Bank Debt (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Revolving credit facility		$ 400,000,000
Proceeds From New Credit Agreement			250,613,000
Revolving credit facility, with option to increase		500,000,000
Maturity date of bank borrowings		May 27, 2016
Aggregate bank borrowings		134,200,000	131,800,000
Revolving credit facility, available borrowings		$ 245,000,000
Maximum consolidated leverage ratio	3.25	3.50
Minimum consolidated fixed charge coverage ratio		2.00
Consolidated leverage ratio		1.3
Consolidated fixed charge coverage ratio		4.4
Credit Agreement, Average Borrowing Rate		2.70%	3.00%
LIBOR Loans [Member]
Debt facility, applicable margin.		1.25%
Base Rate Loans [Member]
Debt facility, applicable margin.		0.25%
Maximum [Member] | LIBOR Loans [Member]
Debt facility, applicable margin.		2.00%
Maximum [Member] | Base Rate Loans [Member]
Debt facility, applicable margin.		1.00%
Minimum [Member] | LIBOR Loans [Member]
Debt facility, applicable margin.		1.00%
Minimum [Member] | Base Rate Loans [Member]
Debt facility, applicable margin.		0.00%