Selected Quarterly Financial Data (unaudited) (Details) (USD $) In Millions, except Per 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
Selected Quarterly Financial Data (unaudited)
Revenues	$ 401.4		$ 397.2		$ 365.8		$ 286.6		$ 356.6		$ 353.2		$ 338.9		$ 272.3		$ 1,451.0	$ 1,321.0		$ 1,243.2
Net income	$ 58.5		$ 51.3		$ 53.1		$ 42.1		$ 53.3		$ 53.8		$ 35.1		$ 27.1		$ 205.0	$ 169.3		$ 137.3
Share Data
Basic earnings per share (in dollars per share)	$ 1.32	[1]	$ 1.16	[1]	$ 1.17	[1]	$ 0.92	[1]	$ 1.16	[1]	$ 1.16	[1]	$ 0.75	[1]	$ 0.58	[1]	$ 4.55	$ 3.64	[1]	$ 2.93
Diluted earnings per share (in dollars per share)	$ 1.30	[1]	$ 1.14	[1]	$ 1.15	[1]	$ 0.90	[1]	$ 1.14	[1]	$ 1.15	[1]	$ 0.74	[1]	$ 0.57	[1]	$ 4.48	$ 3.59	[1]	$ 2.88

[1] (1)Quarterly earnings per share may not be additive, as per share amounts are computed independently for each quarter and the full year is based on the respective weighted average common shares and common stock equivalents outstanding.