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Significant Accounting Policies - Schedule of Premium Revenue (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Summarized premium revenue      
Premium revenue $ 18,854 $ 16,445 $ 13,261
Health Plans      
Summarized premium revenue      
Premium revenue $ 18,854 $ 16,445 $ 13,261
Premium revenue percentage 100.00% 100.00% 100.00%
Health Plans | California      
Summarized premium revenue      
Premium revenue $ 2,701 $ 2,378 $ 2,200
Premium revenue percentage 14.30% 14.40% 16.60%
Health Plans | Florida      
Summarized premium revenue      
Premium revenue $ 2,568 $ 1,938 $ 1,199
Premium revenue percentage 13.60% 11.80% 9.10%
Health Plans | Illinois      
Summarized premium revenue      
Premium revenue $ 593 $ 603 $ 398
Premium revenue percentage 3.10% 3.70% 3.00%
Health Plans | Michigan      
Summarized premium revenue      
Premium revenue $ 1,596 $ 1,527 $ 1,072
Premium revenue percentage 8.50% 9.30% 8.10%
Health Plans | New Mexico      
Summarized premium revenue      
Premium revenue $ 1,368 $ 1,305 $ 1,237
Premium revenue percentage 7.30% 7.90% 9.30%
Health Plans | New York      
Summarized premium revenue      
Premium revenue $ 181 $ 82 $ 0
Premium revenue percentage 1.00% 0.50% 0.00%
Health Plans | Ohio      
Summarized premium revenue      
Premium revenue $ 2,216 $ 1,967 $ 2,035
Premium revenue percentage 11.80% 12.00% 15.30%
Health Plans | Puerto Rico      
Summarized premium revenue      
Premium revenue $ 732 $ 726 $ 567
Premium revenue percentage 3.90% 4.40% 4.30%
Health Plans | South Carolina      
Summarized premium revenue      
Premium revenue $ 445 $ 378 $ 348
Premium revenue percentage 2.40% 2.30% 2.60%
Health Plans | Texas      
Summarized premium revenue      
Premium revenue $ 2,813 $ 2,461 $ 1,963
Premium revenue percentage 14.90% 15.00% 14.80%
Health Plans | Utah      
Summarized premium revenue      
Premium revenue $ 535 $ 447 $ 334
Premium revenue percentage 2.80% 2.70% 2.50%
Health Plans | Washington      
Summarized premium revenue      
Premium revenue $ 2,608 $ 2,222 $ 1,605
Premium revenue percentage 13.80% 13.50% 12.10%
Health Plans | Wisconsin      
Summarized premium revenue      
Premium revenue $ 491 $ 398 $ 262
Premium revenue percentage 2.60% 2.40% 2.00%
Health Plans | Other      
Summarized premium revenue      
Premium revenue $ 7 $ 13 $ 41
Premium revenue percentage 0.00% 0.10% 0.30%