Summary of Significant Accounting Policies - Medical Costs, Advertising, and Accrued Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cost of Care, Medical Claims Payable and Other Medical Liabilities
Period considered for comparing medical claims trend									2 months
Minimum completion factor of insured claims to make projection from historical completion and payment patterns (as a percent)									70.00%
Changes in medical claims payable
Claims payable and IBNR, beginning of period				$ 278,803				$ 242,229	$ 278,803	$ 242,229	$ 222,929
Cost of care:
Current year									2,297,255	2,097,395	2,264,276
Prior years									(22,500)	(8,800)	(31,300)
Total cost of care	$ 587,816	$ 596,323	$ 568,288	$ 522,328	$ 506,090	$ 495,180	$ 481,617	$ 605,708	2,274,755	2,088,595	2,232,976
Claim payments and transfers to other medical liabilities:
Current year									2,077,729	1,845,325	2,053,274
Prior years									222,530	206,696	160,402
Total claim payments and transfers to other medical liabilities									2,300,259	2,052,021	2,213,676
Claims payable and IBNR, end of period	253,299				278,803				253,299	278,803	242,229
Withhold receivables, end of period	(2,850)				(321)				(2,850)	(321)	(13,888)
Medical claims payable, end of period	250,449				278,482				250,449	278,482	228,341
Prior years cost of care claims relating to annual reconciliation process with certain providers											15,100
Prior years cost of care claims relating to adjustments new contracts to providers											8,300
Prior years cost of care claims relating to adjustments faster claims to providers											7,900
Advertising Costs
Advertising expense									$ 2,500	$ 2,700	$ 2,300
Accrued Liabilities
Disclosure threshold (as a percent)									5.00%	5.00%
Accrued employee compensation liabilities	$ 37,600				47,000				$ 37,600	$ 47,000
Deferred revenue					$ 29,800					$ 29,800