Quarterly Financial Data (unaudited) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	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	Dec. 31, 2010	Dec. 31, 2009
Quarterly Financial Data (unaudited)
Premium revenues	$ 1,641,695	$ 1,600,502	$ 1,523,433	$ 1,535,795	$ 1,497,928	$ 1,489,884	$ 1,428,879	$ 1,366,767	$ 6,301,425	$ 5,783,458	$ 5,158,989
Health benefits expenses	1,390,889	1,342,648	1,281,760	1,256,962	1,204,383	1,199,706	1,176,445	1,141,572	5,272,259	4,722,106	4,407,273
Selling, general and administrative expenses	145,271	130,785	122,289	116,459	119,642	106,815	108,189	117,423	514,804	452,069	394,089
Income before income taxes	51,111	76,514	69,444	112,777	126,568	135,338	106,783	68,482	309,846	437,171	201,419
Net income	$ 32,776	$ 48,074	$ 44,294	$ 70,477	$ 79,628	$ 84,348	$ 67,213	$ 42,182	$ 195,621	$ 273,371	$ 149,279
Diluted net income per share	$ 0.67	$ 0.96	$ 0.83	$ 1.37	$ 1.59	$ 1.68	$ 1.31	$ 0.82	$ 3.82	$ 5.40	$ 2.85
Weighted average number of common shares and dilutive potential common shares outstanding	49,179,891	50,253,757	53,541,368	51,534,794	49,924,608	50,197,740	51,318,044	51,226,435	51,163,108	50,608,008	52,309,268