Activity in Benefits Payable (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balances at January 1		$ 4,475	$ 3,893	$ 3,775
Less: Reinsurance recoverables	$ (85)	(85)	(78)	0	$ 0
Balances at January 1, net			4,397	3,893	3,775
Acquisitions		0	0	5
Current year, Incurred		44,397	38,641	32,711
Prior years, Incurred		(236)	(518)	(474)
Total incurred		44,161	38,123	32,237
Current year, Paid		(39,802)	(34,357)	(29,103)
Prior years, Paid		(4,041)	(3,262)	(3,021)
Total paid		(43,843)	(37,619)	(32,124)
Provision for premium deficiency	176	176	0	0
Reinsurance recoverable	(85)	(85)	(78)	0	$ 0
Balances at December 31	$ 4,976	$ 4,976	$ 4,475	$ 3,893