Stockholders' Equity (Details 4) (USD $)	1 Months Ended						2 Months Ended		3 Months Ended		6 Months Ended	10 Months Ended	12 Months Ended
	Oct. 31, 2011	Feb. 28, 2011	Jan. 31, 2011 Y	Jul. 31, 2010	Jul. 31, 2009	Jul. 31, 2008	Dec. 31, 2011	Dec. 31, 2010	Apr. 02, 2010	Apr. 07, 2009	Dec. 31, 2009	Nov. 10, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Stock Repurchases
Amount authorized under stock repurchase plan	$ 200,000,000			$ 350,000,000	$ 100,000,000	$ 200,000,000
Number of shares repurchased (in shares)							671,776	1,684,510	1,711,881	1,859,959	782,400	7,534,766				3,866,505
Average price of shares repurchased (in dollars per share)							$ 48.72	$ 48.36	$ 43.46	$ 34.39	$ 32.75	$ 48.91
Aggregate cost of shares repurchased							32,700,000	81,500,000	74,400,000	64,000,000	25,600,000	368,500,000	401,514,000	155,935,000	89,667,000	136,000,000
Increase in amount authorized under stock repurchase plan		100,000,000							100,000,000	200,000,000
Recent Sales of Unregistered Securities
Number of common stock shares purchased by Blue Shield under share purchase agreement			416,840
Purchase price of common stock acquired by Blue Shield under share purchase agreement			$ 20,000,000
Period for which common stock acquired by Blue Shield cannot be transferred under share purchase agreement (in years)			2