Net Income Per Share and Stockholders' Equity Net Income Per Share and Stockholders' Equity, Share Repurchases (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Number of common shares repurchased (shares)	617,151	938,680	443,838
Weighted-average repurchase price (dollars per share)	$ 100.28	$ 82.97	$ 78.59
Total cost of repurchased common shares	$ 61.9	$ 77.8	$ 34.9

Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

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