Schedule I - Parent Company Financial Information (Condensed Statements of Cash Flows) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 1,936	$ 868	$ 1,618
Cash flows from investing activities:
Proceeds from sale of business	0	1,061	72
Purchases of investment securities	(6,566)	(6,739)	(2,883)
Proceeds from sale of investment securities	4,312	5,493	2,409
Maturities of investment securities	1,426	1,065	885
Purchases of property and equipment, net	(527)	(523)	(528)
Net cash (used in) provided by investing activities	(1,362)	320	(63)
Cash flows from financing activities:
Proceeds from issuance of senior notes, net	0	0	1,733
Repayments of Long-term Debt	0	0	(500)
Change in book overdraft	(89)	(33)	(69)
Common stock repurchases	(104)	(385)	(872)
Dividends paid	(177)	(172)	(172)
Tax benefit from stock-based compensation	0	15	12
Proceeds from stock option exercises and other	11	21	29
Net cash provided by (used in) financing activities	732	(552)	(758)
Increase in cash and cash equivalents	1,306	636	797
Cash and cash equivalents at beginning of year	2,571	1,935	1,138
Cash and cash equivalents at end of year	3,877	2,571	1,935
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	1,848	953	1,499
Cash flows from investing activities:
Proceeds from sale of business	0	1,055	0
Capital contributions to operating subsidiaries	(895)	(833)	(442)
Purchases of investment securities	(151)	(507)	(629)
Proceeds from sale of investment securities	25	18	606
Maturities of investment securities	143	108	149
Purchases of property and equipment, net	(382)	(378)	(380)
Net cash (used in) provided by investing activities	(1,260)	(537)	(696)
Cash flows from financing activities:
Proceeds from issuance of senior notes, net	0	0	1,733
(Repayments) proceeds from issuance of commercial paper, net	(2)	298	0
Repayments of Long-term Debt	0	0	(500)
Change in book overdraft	5	(16)	(5)
Common stock repurchases	(104)	(385)	(872)
Dividends paid	(177)	(172)	(172)
Tax benefit from stock-based compensation	0	15	12
Proceeds from stock option exercises and other	11	22	51
Net cash provided by (used in) financing activities	(267)	(238)	247
Increase in cash and cash equivalents	321	178	1,050
Cash and cash equivalents at beginning of year	1,389	1,211	161
Cash and cash equivalents at end of year	$ 1,710	$ 1,389	$ 1,211