Summary of Significant Accounting Policies (Details 8) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
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				$ 222,929,000				$ 157,099,000	$ 222,929,000	$ 157,099,000	$ 166,095,000
Cost of care:
Current year									2,264,276,000	2,076,190,000	1,790,124,000
Prior years									(31,300,000)	(4,300,000)	(5,400,000)
Total cost of care	605,782,000	564,537,000	537,630,000	525,027,000	528,529,000	516,238,000	521,830,000	505,293,000	2,232,976,000	2,071,890,000	1,784,724,000
Claim payments and transfers to other medical liabilities:
Current year									2,053,274,000	1,877,459,000	1,657,291,000
Prior years									160,402,000	128,601,000	136,429,000
Total claim payments and transfers to other medical liabilities									2,213,676,000	2,006,060,000	1,793,720,000
Claims payable and IBNR, end of period	242,229,000				222,929,000				242,229,000	222,929,000	157,099,000
Withhold receivables, end of period	(13,888,000)				(24,500,000)				(13,888,000)	(24,500,000)	(19,126,000)
Medical claims payable, end of period	228,341,000				198,429,000				228,341,000	198,429,000	137,973,000
Prior years cost of care claims relating to adjustments of block funding to providers									15,100,000
Accrued Liabilities
Disclosure threshold (as a percent)									5.00%	5.00%
Accrued employee compensation liabilities	40,200,000				36,500,000				40,200,000	36,500,000
Share-based compensation
Stock compensation expense	$ 6,488,000	$ 4,524,000	$ 4,602,000	$ 5,638,000	$ 3,848,000	$ 4,468,000	$ 4,365,000	$ 5,102,000	$ 21,252,000	$ 17,783,000	$ 17,418,000
Estimated forfeitures (as a percent)									4.00%	4.00%	4.00%
Vesting period									3 years