FORM 10-K

49 earnings methods, which utilize significant unobservable inputs, or Level 3 inputs, as defined by the fair value hierarchy. Under these valuation methods, we are required to make estimates and assumptions about sales, operating margins, growth rates, royalty rates and discount rates based on anticipated cash flows and marketplace data. Approximately $5 million of the goodwill is deductible for tax purposes. 2016 and 2015 Acquisitions In 2016, we paid $186 million in cash and assumed debt of $19 million to acquire six businesses, net of cash acquired and holdbacks. Our acquisition of Able Engineering and Component Services, Inc. and Able Aerospace, Inc. in the first quarter of 2016 represented the largest of these businesses and is included in the Textron Aviation segment. During 2015, we made aggregate cash payments for acquisitions of $81 million, which included three businesses within our Industrial and Textron Aviation segments. Goodwill The changes in the carrying amount of goodwill by segment are as follows: (In millions) Textron Aviation Bell Textron Systems Industrial Total Balance at January 2, 2016 $ 560 $ 31 $ 1,051 $ 381 $ 2,023 Acquisitions 54 — 36 7 97 Foreign currency translation (1) — — (6) (7) Balance at December 31, 2016 613 31 1,087 382 2,113 Acquisitions — — — 234 234 Foreign currency translation 1 — — 16 17 Balance at December 30, 2017 $ 614 $ 31 $ 1,087 $ 632 $ 2,364 Intangible Assets Our intangible assets are summarized below: December 30, 2017 December 31, 2016 (Dollars in millions) Weighted-Average Amortization Period (in years) Gross Carrying Amount Accumulated Amortization Net Gross Carrying Amount Accumulated Amortization Net Patents and technology 14 $ 545 $ (188) $ 357 $ 537 $ (158) $ 379 Customer relationships and contractual agreements 15 418 (255) 163 384 (226) 158 Trade names and trademarks 15 284 (40) 244 264 (36) 228 Other 9 18 (17) 1 18 (16) 2 Total $ 1,265 $ (500) $ 765 $ 1,203 $ (436) $ 767 Trade names and trademarks in the table above include $222 million and $204 million of indefinite-lived intangible assets at December 30, 2017 and December 31, 2016, respectively. Amortization expense totaled $69 million, $66 million and $61 million in 2017, 2016 and 2015, respectively. Amortization expense is estimated to be approximately $68 million, $66 million, $61 million, $58 million and $58 million in 2018, 2019, 2020, 2021 and 2022, respectively.