Stockholders' Equity (Details) (USD $)	0 Months Ended			1 Months Ended	3 Months Ended		12 Months Ended
	Nov. 30, 2012	Nov. 16, 2012	Feb. 21, 2012	Dec. 31, 2012	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount		$ 100,000,000
Treasury Stock, Shares, Acquired	256,262			873,146			1,129,408
Treasury Stock Acquired, Average Cost Per Share	$ 21.58			$ 24.26			$ 23.65
Stock Repurchase Program, Remaining Authorized Repurchase Amount	94,470,000			73,268,000
Gross proceeds from the issuance of common stock			225,000,000
Stock issued during period			6,122,000
Sale of stock, price per share			$ 36.75
Proceeds from sale of common stock, net of issuance costs			$ 218,961,000		$ 0	$ 219,084,000