Net Income (Loss) Per Share and Stockholders' Equity, Narrative (Details) $ / shares in Units, $ in Millions		12 Months Ended
	Apr. 07, 2016	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Apr. 22, 2016 shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares				$ 60.70
Payment of cash dividends to stockholders | $		$ 35.0	$ 32.4	$ 28.1
Payment of quarterly cash dividends | $ / shares		$ 2.00	$ 1.80	$ 1.60
Repurchase of common shares pursuant to an authorization from the Board, shares		938,680	443,838	647,520
Weighted-average price of repurchases of common shares | $ / shares		$ 82.97	$ 78.59	$ 76.35
Repurchase of common shares pursuant to an authorization from the Board, value | $		$ 77.8	$ 34.9	$ 49.4
Common shares available for additional share repurchase | $		$ 110.5	$ 88.4
Preferred Stock, Shares Authorized		5,000,000	5,000,000
Convertible Notes | Convertible Notes Payable
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock Issued During Period, Shares, Other			1,015,185
Warrant Transactions
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of common shares underlying the Warrants outstanding				3,700,000
Series A Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Preferred Stock, Rights Conversion Ratio into Preferred Stock	1
Preferred Stock, Shares Authorized					900,000