. 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. .