Stockholder's Equity (Details) (USD $) In Millions, except Per Share data	1 Months Ended		3 Months Ended	6 Months Ended
	May 31, 2011	Feb. 28, 2011	Jun. 30, 2011	Jun. 30, 2011
Stockholders' Equity
Repurchase of common stock, authorized amount	$ 200	$ 150
Stock repurchased during the period, shares			5.3
Weighted average price per repurchased share			$ 22.67
Stock repurchased during the period, value			119.0
Remaining authorized amount available for stock repurchase				$ 121.9