Schedule of Income subject to domestic and foreign taxation (Detail) (USD $) 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
Income Before Tax United States - Federal									$ 83.2	$ 1,124.7	$ 1,065.2
Income Before Tax Foreign									174.0	206.6	227.1
Income Before Tax Total	(704.3)	293.3	338.0	330.2	335.5	326.5	308.1	361.2	257.2	1,331.3	1,292.3
Current Tax Expense United States - Federal									218.4	246.9	283.7
Current Tax Expense Foreign									12.1	54.1	94.2
Current Tax Expense Total									230.5	301.0	377.9
Deferred Tax Expense (Benefit) United States - Federal									(230.5)	148.5	91.4
Deferred Tax Expense (Benefit) Foreign									21.8	(4.3)	(29.6)
Deferred Tax Expense (Benefit) Total									(208.7)	144.2	61.8
Income Tax Expense (Benefit)									$ 21.8	$ 445.2	$ 439.7