Provision for Income Taxes Using Federal Statutory Rate (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Income tax provision at federal statutory rate	$ 851	$ 759	$ 672
States, net of federal benefit, and Puerto Rico	44	48	32
Tax exempt investment income	(24)	(27)	(26)
Health insurer fee	314	204	0
Nondeductible executive compensation	18	22	6
Concentra sale	(67)	0	0
Other, net	19	17	6
Provision for income taxes	$ 1,155	$ 1,023	$ 690