Long-term Debt (Details) (USD $)	3 Months Ended			1 Months Ended	3 Months Ended	1 Months Ended					3 Months Ended		1 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Senior Notes due April 2021	Mar. 31, 2013 Senior Notes due April 2021	Mar. 31, 2013 Senior Notes due April 2021 Prior to April 1, 2016 item	Mar. 31, 2013 Senior Notes due April 2021 Twelve month period beginning April 1, 2017	Mar. 31, 2013 Senior Notes due April 2021 Twelve month period beginning April 1, 2018	Mar. 31, 2013 Senior Notes due April 2021 Beginning April 1, 2019 and thereafter	Mar. 31, 2013 Senior Secured Credit Agreement	Mar. 31, 2013 Senior Secured Credit Agreement	Mar. 31, 2012 Senior Secured Credit Agreement	Mar. 31, 2013 Revolving credit facility	Mar. 31, 2012 Revolving credit facility	Dec. 31, 2012 Revolving credit facility due November 2015	Mar. 31, 2013 Revolving credit facility due May 2018	Mar. 31, 2013 Term loan facility	Dec. 31, 2012 Term loan facility due November 2015	Mar. 31, 2013 Term loan facility due May 2018
Long-term debt
Long-term debt	$ 732,548,000		$ 680,500,000	$ 344,548,000	$ 344,548,000										$ 530,500,000	$ 238,000,000		$ 150,000,000	$ 150,000,000
Increase (decrease) in current borrowing capacity													(100,000,000)	200,000,000
Maximum borrowing capacity													650,000,000	750,000,000
Interest rate (as a percent)				6.00%	6.00%
Reduction in applicable margins																	0.50%
Decrease in applicable margin (as a percent)													0.25%
Maximum total debt to EBITDA ratio										525.00%
Maximum revised total debt to EBITDA ratio										550.00%
Write-off of deferred financing costs										700,000
Transaction costs					7,600,000						4,300,000	500,000
Revised total debt to EBITDA ratio, applicable period after acquisition										6 months
Maximum senior secured debt to EBITDA ratio										400.00%
Undrawn capacity under revolving credit facility													412,000,000
Available borrowings under revolving credit facility													412,000,000
Principle amount of senior notes				350,000,000	350,000,000
Net proceeds from borrowings of long-term debt after original issuance discount and issuance costs	799,036,000	565,000,000		336,900,000
Original issuance discount				5,500,000	5,500,000
Effective interest rate (as a percent)				6.25%	6.25%
Maximum period in which the entity is required to register the Notes				365 days
Ownership percentage (as a percent)	100.00%
Amount of independent assets or operations	$ 0
Maximum percentage of the aggregate principal amount of debt instruments that may be redeemed prior to 2016						35.00%
Number of equity offerings						1
Redemption price as percentage of principal amount						106.00%	103.00%	101.50%	100.00%
Minimum notes remaining outstanding if notes redeemed prior to 2016 (as a percent)						65.00%
Maximum number of days within which the entity may redeem a percentage of the note following the date of the closing of equity offering						180 days