Schedule of Income subject to domestic and foreign taxation (Detail) (USD $) In Millions, 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
Income Before Tax United States - Federal									$ 1,128.4	$ 160.5	$ 1,113.1
Income Before Tax Foreign									121.1	172.8	206.8
Income Before Tax Total	322.7	320.4	296.4	310.0	(617.3)	288.7	334.5	327.4	1,249.5	333.3	1,319.9
Current Tax Expense United States - Federal									164.4	218.4	246.9
Current Tax Expense Foreign									42.2	12.1	54.1
Current Tax Expense Total									206.6	230.5	301.0
Deferred Tax Expense (Benefit) United States - Federal									173.5	(203.4)	144.5
Deferred Tax Expense (Benefit) Foreign									(25.0)	22.0	(4.3)
Income Tax (Benefit) - Deferred									148.5	(181.4)	140.2
Income Tax Expense (Benefit)									$ 355.1	$ 49.1	$ 441.2