note 9 : shareholders 2019 equity common stock under the dividend reinvestment and direct stock purchase plan ( the 201cdrip 201d ) , shareholders may reinvest cash dividends and purchase additional company common stock , up to certain limits , through the plan administrator without commission fees . shares purchased by participants through the drip may be newly issued shares , treasury shares , or at the company 2019s election , shares purchased by the plan administrator in the open market or in privately negotiated transactions . purchases generally will be made and credited to drip accounts once each week . as of december 31 , 2018 , there were approximately 4.2 million shares available for future issuance under the drip . anti-dilutive stock repurchase program in february 2015 , the company 2019s board of directors authorized an anti-dilutive stock repurchase program , which allowed the company to purchase up to 10 million shares of its outstanding common stock over an unrestricted period of time . the company repurchased 0.6 million shares and 0.7 million shares of common stock in the open market at an aggregate cost of $ 45 million and $ 54 million under this program for the years ended december 31 , 2018 and 2017 , respectively . as of december 31 , 2018 , there were 5.5 million shares of common stock available for purchase under the program. .
intangible asset amortization expense amounted to $ 12 million , $ 4 million and $ 4 million for the years ended december 31 , 2018 , 2017 and 2016 , respectively . estimated amortization expense for the next five years subsequent to december 31 , 2018 is as follows: .
the company repurchased how many million shares of common stock in the open market for the years ended december 31 , 2018 and 2017?
the company repurchased 0.6 million shares and 0.7 million shares of common stock in the open market at an aggregate cost of $ 45 million and $ 54 million under this program for the years ended december 31 , 2018 and 2017 , respectively .
amount
2019 $ 15
2020 13
2021 11
2022 10
2023 7