Summary of Significant Accounting Policies - Medical Claims Payable and Accrued Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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				$ 188,618				$ 253,299	$ 188,618	$ 253,299	$ 278,803
Cost of care:
Current year									2,421,270	1,892,914	2,297,255
Prior years									(7,500)	(10,300)	(22,500)
Total cost of care	$ 779,146	$ 569,306	$ 583,264	$ 482,054	$ 472,211	$ 480,243	$ 472,529	$ 457,631	2,413,770	1,882,614	2,274,755
Claim payments and transfers to other medical liabilities:
Current year									2,210,346	1,733,310	2,077,729
Prior years									161,798	213,985	222,530
Total claim payments and transfers to other medical liabilities									2,372,144	1,947,295	2,300,259
Acquisition of SWH	96,398								96,398
Claims payable and IBNR, end of period	326,642				188,618				326,642	188,618	253,299
Withhold (receivables), end of period					(4,482)					(4,482)	(2,850)
Withhold payable, end of period	983								983
Medical claims payable, end of period	327,625				184,136				327,625	184,136	$ 250,449
Accrued Liabilities
Accrued employee compensation liabilities	$ 45,600				$ 76,100				$ 45,600	$ 76,100