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Net Income Per Common Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Income Per Common Share [Abstract]
Net income	$ 56.2us-gaap_NetIncomeLoss	$ 71.1us-gaap_NetIncomeLoss	$ 41.9us-gaap_NetIncomeLoss	$ 50.1us-gaap_NetIncomeLoss	$ 65.0us-gaap_NetIncomeLoss	$ 59.0us-gaap_NetIncomeLoss	$ 57.7us-gaap_NetIncomeLoss	$ 46.8us-gaap_NetIncomeLoss	$ 219.3us-gaap_NetIncomeLoss	$ 228.5us-gaap_NetIncomeLoss	$ 138.5us-gaap_NetIncomeLoss
Basic weighted-average common shares outstanding									43,018,215	44,243,364	44,283,771
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Diluted weighted-average common shares outstanding									43,455,136	44,527,405	44,359,891
Net income per basic common share	$ 1.33us-gaap_EarningsPerShareBasic	$ 1.66us-gaap_EarningsPerShareBasic	$ 0.97us-gaap_EarningsPerShareBasic	$ 1.14us-gaap_EarningsPerShareBasic	$ 1.48us-gaap_EarningsPerShareBasic	$ 1.33us-gaap_EarningsPerShareBasic	$ 1.3us-gaap_EarningsPerShareBasic	$ 1.05us-gaap_EarningsPerShareBasic	$ 5.1us-gaap_EarningsPerShareBasic	$ 5.16us-gaap_EarningsPerShareBasic	$ 3.13us-gaap_EarningsPerShareBasic
Net income per diluted common share	$ 1.32us-gaap_EarningsPerShareDiluted	$ 1.65us-gaap_EarningsPerShareDiluted	$ 0.96us-gaap_EarningsPerShareDiluted	$ 1.13us-gaap_EarningsPerShareDiluted	$ 1.46us-gaap_EarningsPerShareDiluted	$ 1.32us-gaap_EarningsPerShareDiluted	$ 1.3us-gaap_EarningsPerShareDiluted	$ 1.05us-gaap_EarningsPerShareDiluted	$ 5.05us-gaap_EarningsPerShareDiluted	$ 5.13us-gaap_EarningsPerShareDiluted	$ 3.12us-gaap_EarningsPerShareDiluted
Antidilutive shares not included in net income per diluted common share calculation									50,236	292,750	2,220,045
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dilutive shares from share based compensation									342,791	194,978	74,232
Stock Award Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dilutive shares from share based compensation									94,130	89,063	1,888