Activity in Benefits Payable (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Liability for Unpaid Claims and Claims Adjustment Expense, Adjustment of Opening Balance [Line Items]
Balances at January 1	$ 3,415	$ 3,214	$ 2,943
Acquisitions	66	29	0
Current year, Incurred	30,198	25,834	24,226
Prior years, Incurred	(257)	(372)	(434)
Total incurred	29,941	25,462	23,792
Current year, Paid	(26,738)	(22,742)	(21,309)
Prior years, Paid	(2,909)	(2,548)	(2,212)
Total paid	(29,647)	(25,290)	(23,521)
Balances at December 31	$ 3,775	$ 3,415	$ 3,214