Net Income Per Share and Stockholders' Equity, Share Repurchases (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Number of common shares repurchased (shares)	304,440	806,307
Weighted-average repurchase price (in usd per share)	$ 106.00	$ 80.60
Total cost of repurchased common shares	$ 32.3	$ 64.9

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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