Summary of Significant Accounting Policies - Medical Costs, Advertising, and Accrued Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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				$ 253,299				$ 278,803	$ 253,299	$ 278,803	$ 242,229
Cost of care:
Current year									1,892,914	2,297,255	2,097,395
Prior years									(10,300)	(22,500)	(8,800)
Total cost of care	$ 472,211	$ 480,243	$ 472,529	$ 457,631	$ 587,816	$ 596,323	$ 568,288	$ 522,328	1,882,614	2,274,755	2,088,595
Claim payments and transfers to other medical liabilities:
Current year									1,733,310	2,077,729	1,845,325
Prior years									213,985	222,530	206,696
Total claim payments and transfers to other medical liabilities									1,947,295	2,300,259	2,052,021
Claims payable and IBNR, end of period	188,618				253,299				188,618	253,299	278,803
Withhold receivables, end of period	(4,482)				(2,850)				(4,482)	(2,850)	(321)
Medical claims payable, end of period	184,136				250,449				184,136	250,449	278,482
Advertising Costs
Advertising expense									2,000	2,500	$ 2,700
Accrued Liabilities
Accrued employee compensation liabilities	$ 76,100				$ 37,600				$ 76,100	$ 37,600