Significant Accounting Policies - Schedule of Premium Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 16,208	$ 17,612	$ 18,854
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	2,062
Health Plans
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 16,208	$ 17,612	$ 18,854
Premium revenue percentage	100.00%	100.00%	100.00%
Health Plans | California
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 2,266	$ 2,150	$ 2,701
Premium revenue percentage	14.00%	12.20%	14.30%
Health Plans | Florida
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 734	$ 1,790	$ 2,568
Premium revenue percentage	4.50%	10.20%	13.60%
Health Plans | Illinois
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 1,002	$ 793	$ 593
Premium revenue percentage	6.20%	4.50%	3.10%
Health Plans | Michigan
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 1,624	$ 1,601	$ 1,596
Premium revenue percentage	10.00%	9.10%	8.50%
Health Plans | New Mexico
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 0	$ 1,356	$ 1,368
Premium revenue percentage	0.00%	7.70%	7.30%
Health Plans | Ohio
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 2,553	$ 2,388	$ 2,216
Premium revenue percentage	15.80%	13.60%	11.80%
Health Plans | Puerto Rico
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 474	$ 696	$ 732
Premium revenue percentage	2.90%	3.90%	3.90%
Health Plans | South Carolina
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 583	$ 495	$ 445
Premium revenue percentage	3.60%	2.80%	2.40%
Health Plans | Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 2,991	$ 3,244	$ 2,813
Premium revenue percentage	18.50%	18.40%	14.90%
Health Plans | Washington
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 2,695	$ 2,361	$ 2,608
Premium revenue percentage	16.60%	13.40%	13.80%
Health Plans | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Premium revenue	$ 1,286	$ 738	$ 1,214
Premium revenue percentage	7.90%	4.20%	6.40%