Schedule of Income subject to domestic and foreign taxation (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax [Abstract]
Income Before Tax United States - Federal									$ 1,057.8	$ 391.7	$ 1,086.8
Income Before Tax Foreign									180.5	150.3	133.2
Income Before Income Tax	$ 316.6	$ 298.9	$ 319.8	$ 303.0	$ (454.6)	$ 317.0	$ 349.8	$ 329.8	1,238.3	542.0	1,220.0
Current Tax Expense United States - Federal									280.5	160.6	309.8
Current Tax Expense Foreign									61.6	(25.1)	18.7
Income Tax - Current									342.1	135.5	328.5
Deferred Tax Expense (Benefit) United States - Federal									56.9	(50.5)	41.3
Deferred Tax Expense (Benefit) Foreign									(27.8)	54.9	3.2
Income Tax - Deferred									29.1	4.4	44.5
Income Tax Expense									$ 371.2	$ 139.9	$ 373.0