Supplemental Condensed Consolidating Financial Information - Condensed Consolidating Statements of Cash Flow (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Operating activities:
Net cash provided by operating activities	$ 633	$ 906
Investing activities:
Purchases of investments	(1,444)	(1,311)
Proceeds from sales and maturities of investments	1,512	863
Purchases of property, equipment and capitalized software	(143)	(101)
Change in restricted investments	4	(5)
Net cash paid in business combinations	(48)	(77)
Capital contributions to subsidiaries	0	0
Dividends received from subsidiaries	0	0
Change in amounts due to/from affiliates	0	0
Other, net	(12)	(34)
Net cash used in investing activities	(131)	(665)
Financing activities:
Proceeds from common stock offering, net of issuance costs	0	373
Proceeds from employee stock plans	10	8
Other, net	1	3
Net cash provided by financing activities	11	384
Net increase in cash and cash equivalents	513	625
Cash and cash equivalents at beginning of period	2,329	1,539
Cash and cash equivalents at end of period	2,842	2,164
Eliminations
Operating activities:
Net cash provided by operating activities	0	0
Investing activities:
Purchases of investments	0	0
Proceeds from sales and maturities of investments	0	0
Purchases of property, equipment and capitalized software	0	0
Change in restricted investments	0	0
Net cash paid in business combinations	0	0
Capital contributions to subsidiaries	0	0
Dividends received from subsidiaries	0	0
Change in amounts due to/from affiliates	0	0
Other, net	0	0
Net cash used in investing activities	0	0
Financing activities:
Proceeds from common stock offering, net of issuance costs		0
Proceeds from employee stock plans	0	0
Other, net	0	0
Net cash provided by financing activities	0	0
Net increase in cash and cash equivalents	0	0
Cash and cash equivalents at beginning of period	0	0
Cash and cash equivalents at end of period	0	0
Parent Guarantor | Reportable Legal Entities
Operating activities:
Net cash provided by operating activities	43	93
Investing activities:
Purchases of investments	(114)	(23)
Proceeds from sales and maturities of investments	103	90
Purchases of property, equipment and capitalized software	(102)	(68)
Change in restricted investments	0	0
Net cash paid in business combinations	0	0
Capital contributions to subsidiaries	(221)	(167)
Dividends received from subsidiaries	50	42
Change in amounts due to/from affiliates	(12)	(15)
Other, net	6	(1)
Net cash used in investing activities	(290)	(142)
Financing activities:
Proceeds from common stock offering, net of issuance costs		373
Proceeds from employee stock plans	10	8
Other, net	2	3
Net cash provided by financing activities	12	384
Net increase in cash and cash equivalents	(235)	335
Cash and cash equivalents at beginning of period	360	75
Cash and cash equivalents at end of period	125	410
Other Guarantors | Reportable Legal Entities
Operating activities:
Net cash provided by operating activities	35	70
Investing activities:
Purchases of investments	0	0
Proceeds from sales and maturities of investments	0	0
Purchases of property, equipment and capitalized software	(28)	(19)
Change in restricted investments	0	5
Net cash paid in business combinations	(11)	0
Capital contributions to subsidiaries	18	3
Dividends received from subsidiaries	0	(17)
Change in amounts due to/from affiliates	1	0
Other, net	(19)	(25)
Net cash used in investing activities	(39)	(53)
Financing activities:
Proceeds from common stock offering, net of issuance costs		0
Proceeds from employee stock plans	0	0
Other, net	(1)	0
Net cash provided by financing activities	(1)	0
Net increase in cash and cash equivalents	(5)	17
Cash and cash equivalents at beginning of period	42	15
Cash and cash equivalents at end of period	37	32
Non-Guarantors | Reportable Legal Entities
Operating activities:
Net cash provided by operating activities	555	743
Investing activities:
Purchases of investments	(1,330)	(1,288)
Proceeds from sales and maturities of investments	1,409	773
Purchases of property, equipment and capitalized software	(13)	(14)
Change in restricted investments	4	(10)
Net cash paid in business combinations	(37)	(77)
Capital contributions to subsidiaries	203	164
Dividends received from subsidiaries	(50)	(25)
Change in amounts due to/from affiliates	11	15
Other, net	1	(8)
Net cash used in investing activities	198	(470)
Financing activities:
Proceeds from common stock offering, net of issuance costs		0
Proceeds from employee stock plans	0	0
Other, net	0	0
Net cash provided by financing activities	0	0
Net increase in cash and cash equivalents	753	273
Cash and cash equivalents at beginning of period	1,927	1,449
Cash and cash equivalents at end of period	$ 2,680	$ 1,722