Significant Accounting Policies - 3R Program Receivables (Payables) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Risk adjustment payable, Current Benefit Year	$ (372)
Risk adjustment benefit, Prior Benefit Years	2
Risk adjustment payable, Total	(370)	$ (214)
Reinsurance receivable, Current Benefit Year	62
Reinsurance receivable, Prior Benefit Years	4
Reinsurance receivable, Total	66	36
Risk corridor payable, Current Benefit Year	(4)
Risk corridor payable, Prior Benefit Years	(1)
Risk corridor payable, Total	(5)	(10)
Minimum MLR payable, Current Benefit Year	(11)
Minimum MLR payable, Prior Benefit Years	(4)
Minimum MLR payable, Total	$ (15)	$ (3)