Earnings Per Common Share (Earnings Per Share Computation) (Details) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Common Share [Abstract]
Net income attributable to common shareholders	$ 113,800,000	$ 130,100,000	$ (47,800,000)	$ 131,900,000	$ 136,200,000	$ 127,500,000	$ 124,600,000	$ 120,900,000	$ 328,000,000	$ 509,200,000	$ 460,100,000
Less: Income allocated to participating securities									6,100,000	5,500,000	5,000,000
Net income available to common shareholders									$ 321,900,000	$ 503,700,000	$ 455,100,000
Weighted average common shares outstanding									85.8	93.4	97.7
Dilutive common share equivalents from share-based compensation plans									1.5	1.2	1.3
Weighted average common and common equivalent shares outstanding, assuming dilution									87.3	94.6	99.0