Significant Accounting Policies - Consolidated Medical Care Costs (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Medical Care Costs
Fee-for-service	$ 11,278,000,000	$ 12,682,000,000	$ 10,993,000,000
Pharmacy	2,138,000,000	2,563,000,000	2,213,000,000
Capitation	1,184,000,000	1,360,000,000	1,218,000,000
Other	537,000,000	468,000,000	350,000,000
Total	15,137,000,000	17,073,000,000	14,774,000,000
Medical Care Costs, PMPM
Fee-for-service (per member per month)	232.15	229.63	217.84
Pharmacy (per member per month)	44.01	46.40	43.84
Capitation (per member per month)	24.38	24.63	24.13
Other (per member per month)	11.05	8.48	6.94
Total (per member per month)	$ 311.59	$ 309.14	$ 292.75
Medical Care Costs, Percentage
Percentage of total in Fee for service	74.50%	74.30%	74.40%
Percentage of total in pharmacy	14.10%	15.00%	15.00%
Percentage of total in capitation	7.80%	8.00%	8.20%
Percentage of total in other	3.60%	2.70%	2.40%
Percentage of total	100.00%	100.00%	100.00%