Stockholders' Equity - Share repurchase programs (Details) - USD ($) shares in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2018	Oct. 31, 2017	Jan. 31, 2017
Stockholders' Equity
Number of shares of common stock issued	700
Program Amount authorized to repurchase		$ 100,000	$ 100,000
Repurchase of common stock (in shares)	36
Repurchase of common stock	$ 3,400