Stockholders' Equity (Details 4) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended					2 Months Ended			10 Months Ended	12 Months Ended		0 Months Ended
	Jul. 24, 2013	Oct. 25, 2011	Feb. 18, 2011	Jan. 31, 2011	Jul. 27, 2010	Feb. 26, 2014	Dec. 31, 2011	Dec. 31, 2010	Nov. 10, 2011	Dec. 31, 2013	Dec. 31, 2012	Oct. 02, 2013 Partners Rx
Stock Repurchases
Amount authorized under stock repurchase plan	$ 300	$ 200	$ 450		$ 350
Increase in amount authorized under stock repurchase plan			100
Share repurchases made in open market (in shares)						177,227	671,776	1,684,510	7,534,766	1,159,871	459,252
Average price of shares repurchased (in dollars per share)							$ 48.72	$ 48.36	$ 48.91	$ 51.83	$ 50.27
Aggregate cost of shares repurchased, excluding broker commissions						10.6	32.7	81.5	368.5	60.1	23.1
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
Period for which common stock acquired by Blue Shield cannot be transferred under share purchase agreement				2 years
Merger Agreement
Number of restricted stock purchased by certain principal owners of the acquiree												175,596
Purchase price of restricted stock												$ 10
Trading day period to calculate average of the closing prices of the company's stock												5 days
Vesting period of restricted stock issued												3 years
Vesting percentage on the second anniversary of the acquisition												50.00%
Vesting percentage on the third anniversary of the acquisition												50.00%