Schedule of Income subject to domestic and foreign taxation (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax [Abstract]
Income Before Tax United States - Federal									$ 376.9	$ 1,072.0	$ 1,128.4
Income Before Tax Foreign									150.3	133.2	121.1
Income Before Income Tax	(457.9)	312.1	346.5	326.5	305.8	284.1	311.5	303.8	527.2	1,205.2	1,249.5
Current Tax Expense United States - Federal									128.4	277.9	164.4
Current Tax Expense Foreign									(25.1)	18.7	42.2
Current Tax Expense Total									103.3	296.6	206.6
Deferred Tax Expense (Benefit) United States - Federal									(44.4)	47.3	173.5
Deferred Tax Expense (Benefit) Foreign									54.9	3.2	(25.0)
Income Tax (Benefit) - Deferred									10.5	50.5	148.5
Income Tax Expense (Benefit)									$ 113.8	$ 347.1	$ 355.1