Net Income Per Common Share (Detail) (USD $) In Millions, except Share data, 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
Numerator
Net Income	$ (425.4)	$ 205.6	$ 229.8	$ 225.4	$ 225.8	$ 220.8	$ 209.7	$ 229.8	$ 235.4	$ 886.1	$ 852.6
Denominator (000s)
Weighted Average Common Shares - Basic									302,399,800	325,839,000	331,266,200
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards									1,171,200	1,382,100	870,000
Weighted Average Common Shares - Assuming Dilution									303,571,000	327,221,100	332,136,200
Net Income Per Common Share
Earnings Per Share, Basic	$ (1.45)	$ 0.69	$ 0.75	$ 0.72	$ 0.71	$ 0.68	$ 0.63	$ 0.69	$ 0.78	$ 2.72	$ 2.57
Earnings Per Share, Diluted	$ (1.45)	$ 0.69	$ 0.75	$ 0.72	$ 0.71	$ 0.68	$ 0.63	$ 0.69	$ 0.78	$ 2.71	$ 2.57