Net Income Per Share and Stockholders' Equity, Share Repurchases (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Equity [Abstract]
Number of common shares repurchased (shares)	175,977	58,155
Weighted-average repurchase price (in usd per share)	$ 98.55	$ 104.50
Total cost of repurchased common shares	$ 17.4	$ 6.1

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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