Net Income Per Common Share (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator
Net Income	$ 233.9	$ 230.2	$ 216.4	$ 213.9	$ (369.0)	$ 202.0	$ 227.6	$ 223.6	$ 894.4	$ 284.2	$ 878.7
Denominator (000s)
Weighted Average Common Shares - Basic									281,355,900	302,399,800	325,839,000
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards									400,900	1,171,200	1,382,100
Weighted Average Common Shares - Assuming Dilution									281,756,800	303,571,000	327,221,100
Net Income Per Common Share
Earnings Per Share, Basic	$ 0.86	$ 0.83	$ 0.76	$ 0.74	$ (1.26)	$ 0.68	$ 0.74	$ 0.72	$ 3.18	$ 0.94	$ 2.70
Earnings Per Share, Diluted	$ 0.85	$ 0.83	$ 0.76	$ 0.73	$ (1.26)	$ 0.68	$ 0.74	$ 0.71	$ 3.17	$ 0.94	$ 2.69