Long-Term Debt (Details 3) (USD $)	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2013
Long-Term Debt
Net proceeds from borrowings of long-term debt after original issuance discount and issuance costs	$ 927,798,000	$ 1,005,536,000	$ 714,000,000
Unamortized debt issuance discount	9,700,000
Amortization of debt discount	1,042,000	419,000
6% senior notes due April 2021
Long-Term Debt
Principle amount of senior notes				350,000,000
Interest rate (as a percent)	6.00%	6.00%		6.00%
Net proceeds from borrowings of long-term debt after original issuance discount and issuance costs				336,900,000
Transaction costs		7,800,000
Unamortized debt issuance discount	4,500,000	5,000,000		5,500,000
Effective interest rate (as a percent)	6.25%
Amortization of debt discount	$ 500,000	$ 400,000
6% senior notes due April 2021 | Prior to April 1, 2016
Long-Term Debt
Maximum percentage of the aggregate principal amount of debt instruments that may be redeemed prior to 2017				35.00%
Minimum number of equity offerings				1
Redemption price as percentage of principal amount				106.00%
Minimum notes remaining outstanding if notes redeemed prior to 2017 (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
6% senior notes due April 2021 | Twelve month period beginning April 1, 2017
Long-Term Debt
Redemption price as percentage of principal amount				103.00%
6% senior notes due April 2021 | Twelve month period beginning April 1, 2018
Long-Term Debt
Redemption price as percentage of principal amount				101.50%
6% senior notes due April 2021 | Beginning April 1, 2019 and thereafter
Long-Term Debt
Redemption price as percentage of principal amount				100.00%