Net Income Per Share and Stockholders' Equity, Narrative (Details) $ / shares in Units, $ in Millions		12 Months Ended
	Apr. 07, 2016	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Apr. 22, 2016 shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Payment of cash dividends to stockholders | $		$ 37.7	$ 35.0	$ 32.4
Payment of quarterly cash dividends | $ / shares		$ 2.20	$ 2.00	$ 1.80
Repurchase of common shares pursuant to an authorization from the Board, shares | shares		617,151	938,680	443,838
Weighted-average price of repurchases of common shares | $ / shares		$ 100.28	$ 82.97	$ 78.59
Repurchase of common shares pursuant to an authorization from the Board, value | $		$ 61.9	$ 77.8	$ 34.9
Common shares available for additional share repurchase | $		$ 148.7	$ 110.5
Preferred Stock, Shares Authorized | shares		5,000,000	5,000,000
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 | shares					900,000