Net Income (Loss) Per Share and Stockholders' Equity Net Income (Loss) Per Share and Stockholders' Equity, Share Repurchases (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity, Class of Treasury Stock [Line Items]
Number of common shares repurchased (shares)	938,680	443,838	647,520
Weighted-average repurchase price (dollars per share)	$ 82.97	$ 78.59	$ 76.35
Total cost of repurchased common shares	$ 77.8	$ 34.9	$ 49.4