Significant Accounting Policies - Quality Incentive Premium Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Maximum available quality incentive premium - current period	$ 186	$ 182	$ 150
Amount of quality incentive premium revenue recognized in current period:
Earned current period	156	133	97
Earned prior periods	38	31	10
Total quality incentive premium revenue recognized	$ 194	$ 164	$ 107
Quality incentive premium revenue recognized as a percentage of total premium revenue	1.20%	0.90%	0.60%