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Significant Accounting Policies - Quality Incentive Premium Revenue Recognized (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Premium Revenue by Health Plan Type [Line Items]
Maximum available quality incentive premium - current period	$ 48	$ 36	$ 135	$ 113
Quality incentive premium revenue recognized in current period:
Earned current period	39	24	97	72
Earned prior periods	9	3	32	9
Total	$ 48	$ 27	$ 129	$ 81
Quality incentive premium revenue recognized as a percentage of total premium revenue	1.10%	0.60%	1.00%	0.60%
Minimum \| Select Health Plans
Schedule of Premium Revenue by Health Plan Type [Line Items]
Percentage of additional incremental revenue earned			1.00%
Maximum \| Select Health Plans
Schedule of Premium Revenue by Health Plan Type [Line Items]
Percentage of additional incremental revenue earned			3.00%