Supplemental Condensed Consolidating Financial Information - Condensed Consolidating Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue:
Revenues									$ 17,782	$ 14,178	$ 9,667
Operating expenses:
Medical care costs									14,774	11,794	8,076
Cost of service revenue									485	193	157
General and administrative expenses									1,393	1,146	765
Premium tax expenses									468	397	294
Health insurer fee expenses									217	157	89
Depreciation and amortization									139	104	93
Total operating expenses									17,476	13,791	9,474
Operating income	$ (6)	$ 118	$ 105	$ 89	$ 76	$ 113	$ 116	$ 82	306	387	193
Interest expense									101	66	57
Total other expenses, net									101	65	58
Income before income tax expense									205	322	135
Income tax expense (benefit)									153	179	73
Net (loss) income before equity in earnings of subsidiaries									52	143	62
Equity in net earnings of subsidiaries									0	0	0
Net income	$ (47)	$ 42	$ 33	$ 24	$ 30	$ 46	$ 39	$ 28	52	143	62
Eliminations
Revenue:
Revenues									(1,139)	(977)	(733)
Operating expenses:
Medical care costs									(58)	(50)	(31)
Cost of service revenue									0	0	0
General and administrative expenses									(1,081)	(927)	(702)
Premium tax expenses									0	0	0
Health insurer fee expenses									0	0	0
Depreciation and amortization									0	0	0
Total operating expenses									(1,139)	(977)	(733)
Operating income									0	0	0
Interest expense									0
Total other expenses, net										0	0
Income before income tax expense									0	0	0
Income tax expense (benefit)									0	0	0
Net (loss) income before equity in earnings of subsidiaries									0	0	0
Equity in net earnings of subsidiaries									(119)	(190)	(89)
Net income									(119)	(190)	(89)
Parent Guarantor
Revenue:
Revenues									1,078	931	706
Operating expenses:
Medical care costs									73	55	46
General and administrative expenses									899	797	583
Depreciation and amortization									95	82	73
Total operating expenses									1,067	934	702
Operating income									11	(3)	4
Interest expense									101	66	57
Income tax expense (benefit)									(24)	(21)	(27)
Net (loss) income before equity in earnings of subsidiaries									(66)	(48)	(27)
Equity in net earnings of subsidiaries									118	191	89
Net income									52	143	62
Parent Guarantor | Reportable Legal Entities
Revenue:
Revenues									1,078	931	706
Operating expenses:
Medical care costs									73	55	46
Cost of service revenue									0	0	0
General and administrative expenses									899	797	583
Premium tax expenses									0	0	0
Health insurer fee expenses									0	0	0
Depreciation and amortization									95	82	73
Total operating expenses									1,067	934	702
Operating income									11	(3)	4
Interest expense									101
Total other expenses, net										66	58
Income before income tax expense									(90)	(69)	(54)
Income tax expense (benefit)									(24)	(21)	(27)
Net (loss) income before equity in earnings of subsidiaries									(66)	(48)	(27)
Equity in net earnings of subsidiaries									118	191	89
Net income									52	143	62
Other Guarantors | Reportable Legal Entities
Revenue:
Revenues									556	293	240
Operating expenses:
Medical care costs									54	36	27
Cost of service revenue									443	184	157
General and administrative expenses									48	41	29
Premium tax expenses									0	0	0
Health insurer fee expenses									0	0	0
Depreciation and amortization									12	4	5
Total operating expenses									557	265	218
Operating income									(1)	28	22
Interest expense									0
Total other expenses, net										0	0
Income before income tax expense									(1)	28	22
Income tax expense (benefit)									(4)	9	8
Net (loss) income before equity in earnings of subsidiaries									3	19	14
Equity in net earnings of subsidiaries									1	(1)	0
Net income									4	18	14
Non-Guarantors | Reportable Legal Entities
Revenue:
Revenues									17,287	13,931	9,454
Operating expenses:
Medical care costs									14,705	11,753	8,034
Cost of service revenue									42	9	0
General and administrative expenses									1,527	1,235	855
Premium tax expenses									468	397	294
Health insurer fee expenses									217	157	89
Depreciation and amortization									32	18	15
Total operating expenses									16,991	13,569	9,287
Operating income									296	362	167
Interest expense									0
Total other expenses, net										(1)	0
Income before income tax expense									296	363	167
Income tax expense (benefit)									181	191	92
Net (loss) income before equity in earnings of subsidiaries									115	172	75
Equity in net earnings of subsidiaries									0	0	0
Net income									$ 115	$ 172	$ 75