Significant Accounting Policies - Quality Incentive Premium Revenue Recognized (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Schedule of Premium Revenue by Health Plan Type [Line Items]
Maximum available quality incentive premium - current period	$ 61	$ 45
Quality incentive premium revenue recognized in current period:
Earned current period	44	26
Earned prior periods	12	20
Total	$ 56	$ 46
Quality incentive premium revenue recognized as a percentage of total premium revenue	1.30%	1.20%
Select Health Plans | Minimum
Schedule of Premium Revenue by Health Plan Type [Line Items]
Percentage of additional incremental revenue earned	1.00%
Select Health Plans | Maximum
Schedule of Premium Revenue by Health Plan Type [Line Items]
Percentage of additional incremental revenue earned	4.00%