Schedule I - Parent Company Financial Information (Condensed Statements of Cash Flows) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 4,051	$ 1,936	$ 868
Cash flows from investing activities:
Proceeds from sale of business	0	0	1,061
Purchases of investment securities	(6,265)	(6,566)	(6,739)
Proceeds from sale of investment securities	2,768	4,312	5,493
Maturities of investment securities	1,111	1,426	1,065
Purchases of property and equipment, net	(526)	(527)	(523)
Net cash (used in) provided by investing activities	(2,941)	(1,362)	320
Cash flows from financing activities:
Proceeds from issuance of senior notes, net	1,779	0	0
(Repayments) proceeds from issuance of commercial paper, net	(153)	(2)	298
Repayment of long-term debt	(800)	0	0
Change in book overdraft	(71)	(89)	(33)
Common stock repurchases	(3,365)	(104)	(385)
Dividends paid	(220)	(177)	(172)
Tax benefit from stock-based compensation	0	0	15
Proceeds from stock option exercises and other, net	63	14	23
Net cash (used in) provided by financing activities	(945)	732	(552)
Increase in cash and cash equivalents	165	1,306	636
Cash and cash equivalents at beginning of year	3,877	2,571	1,935
Cash and cash equivalents at end of year	4,042	3,877	2,571
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	2,423	1,848	953
Cash flows from investing activities:
Proceeds from sale of business	0	0	1,055
Capital contributions to operating subsidiaries	(695)	(895)	(833)
Purchases of investment securities	(53)	(151)	(507)
Proceeds from sale of investment securities	0	25	18
Maturities of investment securities	51	143	108
Purchases of property and equipment, net	(359)	(382)	(378)
Net cash (used in) provided by investing activities	(1,056)	(1,260)	(537)
Cash flows from financing activities:
Proceeds from issuance of senior notes, net	1,779	0	0
(Repayments) proceeds from issuance of commercial paper, net	(153)	(2)	298
Repayment of long-term debt	(800)	0	0
Change in book overdraft	3	5	(16)
Common stock repurchases	(3,365)	(104)	(385)
Dividends paid	(220)	(177)	(172)
Tax benefit from stock-based compensation	0	0	15
Proceeds from stock option exercises and other, net	62	11	22
Net cash (used in) provided by financing activities	(2,694)	(267)	(238)
Increase in cash and cash equivalents	(1,327)	321	178
Cash and cash equivalents at beginning of year	1,710	1,389	1,211
Cash and cash equivalents at end of year	$ 383	$ 1,710	$ 1,389