Stockholders' Equity Share repurchase and retirement (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 10, 2016
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value	$ 10.1
Stock Repurchase Program, Authorized Amount		$ 150.0
Stock Repurchased During Period, Shares	228,831
Additional Paid-in Capital [Member]
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value	$ 2.4
Retained Earnings [Member]
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value	$ 7.7