Stockholders' Equity - Share repurchase programs (Details) - USD ($) $ in Thousands, shares in Millions	3 Months Ended
	Mar. 31, 2018	Oct. 31, 2017	Jan. 31, 2017
Stockholders' Equity
Number of shares of common stock issued	0.5
Program Amount authorized to repurchase		$ 100,000	$ 100,000