Summary of Significant Accounting Policies (Details 8) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011 M	Dec. 31, 2010	Dec. 31, 2009
Cost of Care, Medical Claims Payable and Other Medical Liabilities
Period considered for comparing medical claims trend (in months)									2
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				$ 166,095,000				$ 168,851,000	$ 166,095,000	$ 168,851,000	$ 184,422,000
Cost of care:
Current year									1,790,124,000	1,919,785,000	1,771,213,000
Prior years									(5,400,000)	(11,800,000)	(5,900,000)
Total cost of care	461,527,000	448,051,000	441,446,000	433,700,000	491,668,000	467,160,000	472,478,000	476,679,000	1,784,724,000	1,907,985,000	1,765,313,000
Claim payments and transfers to other medical liabilities:
Current year									1,657,291,000	1,777,356,000	1,624,626,000
Prior years									136,429,000	133,385,000	156,258,000
Total claim payments and transfers to other medical liabilities									1,793,720,000	1,910,741,000	1,780,884,000
Claims payable and IBNR, end of period	157,099,000				166,095,000				157,099,000	166,095,000	168,851,000
Withhold receivables, end of period	(19,126,000)				(23,424,000)				(19,126,000)	(23,424,000)	(25,182,000)
Medical claims payable, end of period	137,973,000				142,671,000				137,973,000	142,671,000	143,669,000
Accrued Liabilities
Disclosure threshold (as a percent)									5.00%	5.00%
Accrued employee compensation liabilities	32,700,000				34,500,000				32,700,000	34,500,000
Share-based compensation
Stock compensation expense	$ 4,010,000	$ 4,425,000	$ 4,205,000	$ 4,778,000	$ 3,272,000	$ 3,596,000	$ 3,706,000	$ 4,528,000	$ 17,418,000	$ 15,102,000	$ 19,782,000
Estimated forfeitures (as a percent)									4.00%	5.00%	5.00%