Net Income Per Share and Stockholders' Equity Net Income Per Share and Stockholders' Equity, Share Repurchases (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Equity [Abstract]
Treasury Stock, Shares, Acquired	415,346	83,265
Treasury Stock Acquired, Average Cost Per Share	$ 78.70	$ 76.85
Treasury Stock, Value, Acquired, Cost Method	$ 32.7	$ 6.4