Selected Quarterly Financial Data (unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012		Dec. 31, 2011
Selected Quarterly Financial Data (unaudited)
Revenues	$ 356.6		$ 353.2		$ 338.9		$ 272.3		$ 331.7		$ 331.9		$ 323.1		$ 256.5		$ 1,321.0	$ 1,243.2		$ 1,191.4
Net income	53.3		53.8		35.1		27.1		29.7		53.8		23.5		30.3		169.3	137.3		110.8
Share Data
Basic earnings per share (in dollars per share)	$ 1.16	[1]	$ 1.16	[1]	$ 0.75	[1]	$ 0.58	[1]	$ 0.63	[1]	$ 1.15	[1]	$ 0.50	[1]	$ 0.65	[1]	$ 3.64	$ 2.93	[1]	$ 2.39
Diluted earnings per share (in dollars per share)	$ 1.14	[1]	$ 1.15	[1]	$ 0.74	[1]	$ 0.57	[1]	$ 0.62	[1]	$ 1.13	[1]	$ 0.49	[1]	$ 0.64	[1]	$ 3.59	$ 2.88	[1]	$ 2.35
Share of affiliates' earnings (net of tax)																	75.8	19.6		32.4
Expiration of the applicable statute of limitations																	$ 0.6	$ (15.5)		$ 0

[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.