Stockholders' Equity (Details) (USD $)	0 Months Ended		12 Months Ended
	Nov. 16, 2012	Feb. 21, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 30, 2011
Stockholders Equity [Line Items]
Gross proceeds from the sale of common stock		$ 225,000,000
Sale of common stock, price per share		$ 36.75				$ 31.50
Sale of common stock, value			218,961,000
Net proceeds from sale of common stock				175,229,000
Stock repurchase program, authorized amount	100,000,000
Restricted stock awards, value of shares withheld for taxes			26,732,000	0	0
Common Stock [Member]
Stockholders Equity [Line Items]
Sale of common stock, shares			6,122,449	5,750,000
Sale of common stock, value			$ 6,000	$ 6,000