Long-term Debt (Details) (USD $)	1 Months Ended	3 Months Ended		12 Months Ended			1 Months Ended		12 Months Ended		1 Months Ended						12 Months Ended			12 Months Ended			12 Months Ended			1 Months Ended			12 Months Ended			12 Months Ended
	May 31, 2011	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Nov. 30, 2010 Asset backed securitization facility	Nov. 30, 2010 Interest rate swap agreements	Dec. 31, 2012 Minimum Covenant Requirement	Dec. 31, 2012 Maximum Covenant Requirement	Nov. 30, 2010 Senior secured credit facility	Mar. 31, 2012 Revolving credit facility due November 2015	Mar. 31, 2011 Revolving credit facility due November 2015	Nov. 30, 2010 Revolving credit facility due November 2015	Dec. 31, 2012 Revolving credit facility due November 2015	Dec. 31, 2011 Revolving credit facility due November 2015	Dec. 31, 2012 Revolving credit facility due November 2015 LIBOR	Dec. 31, 2012 Revolving credit facility due November 2015 LIBOR Minimum	Dec. 31, 2012 Revolving credit facility due November 2015 LIBOR Maximum	Dec. 31, 2012 Revolving credit facility due November 2015 Base rate	Dec. 31, 2012 Revolving credit facility due November 2015 Base rate Minimum	Dec. 31, 2012 Revolving credit facility due November 2015 Base rate Maximum	Dec. 31, 2012 Revolving credit facility due November 2015 Prime rate announced by Wells Fargo Bank, National Association	Dec. 31, 2012 Revolving credit facility due November 2015 Federal Funds Effective Rate	Dec. 31, 2012 Revolving credit facility due November 2015 One-month LIBOR	Nov. 30, 2010 Term loan facility due November 2015	Dec. 31, 2012 Term loan facility due November 2015	Dec. 31, 2011 Term loan facility due November 2015	Dec. 31, 2012 Term loan facility due November 2015 LIBOR	Dec. 31, 2012 Term loan facility due November 2015 LIBOR Minimum	Dec. 31, 2012 Term loan facility due November 2015 LIBOR Maximum	Dec. 31, 2012 Term loan facility due November 2015 Base rate	Dec. 31, 2012 Term loan facility due November 2015 Base rate Minimum	Dec. 31, 2012 Term loan facility due November 2015 Base rate Maximum
Long-term debt
Long-term debt				$ 680,500,000	$ 545,500,000										$ 530,500,000	$ 395,500,000											$ 150,000,000	$ 150,000,000
Credit facility term											5 years
Maximum borrowing capacity											550,000,000	750,000,000	550,000,000	400,000,000												150,000,000
Amount of borrowing														304,000,000												150,000,000
Increase in current borrowing capacity												200,000,000	150,000,000
Amount repaid	64,800,000			684,350,000	620,434,000	454,500,000	30,000,000	14,800,000			406,100,000
Unamortized deferred financing costs											200,000
Transaction costs		500,000	1,000,000	1,011,000	1,003,000	4,064,000					4,000,000
Undrawn capacity under revolving credit facility															219,500,000
Available additional borrowings under revolving credit facility															199,400,000
Variable interest rate base																	LIBOR			base rate			prime rate		one-month LIBOR				LIBOR			base rate
Interest rate added to base rate (as a percent)																		2.25%	3.25%		1.25%	2.25%								2.50%	3.50%		1.50%	2.50%
Interest margin added to calculated variable rate (as a percent)																	0.025							0.005	0.010				0.0275
Weighted average annual interest rate (as a percent)															2.80%												3.00%
Ratio of EBITDA to Total Interest Expense									300.00%
Ratio of EBITDA to Total Interest Expense on occurrence of certain events									275.00%
Ratio of Total Debt to EBITDA										475.00%
Ratio of total debt to EBITDA on occurrence of certain events										525.00%
Contractual maturities of long-term debt (excluding interest to be accrued thereon)
2015				680,500,000
Total debt				$ 680,500,000	$ 545,500,000										$ 530,500,000	$ 395,500,000											$ 150,000,000	$ 150,000,000