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Significant Accounting Policies - Quality Incentive Premium Revenue Recognized (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Premium Revenue by Health Plan Type [Line Items]
Maximum available quality incentive premium - current period	$ 47	$ 39	$ 87	$ 77
Quality incentive premium revenue recognized in current period:
Earned current period	34	29	58	48
Earned prior periods	12	1	23	6
Total	$ 46	$ 30	$ 81	$ 54
Quality incentive premium revenue recognized as a percentage of total premium revenue	1.00%	0.60%	0.90%	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%