Debt Debt - Senior Secured Credit Facilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument
Secured Debt	$ 700,000
Unamortized discount and debt issuance costs	9,217	$ 9,516
Long-term Debt, Gross	1,095,000	946,000
Line of Credit Facility, Maximum Borrowing Capacity	$ 300,000
Line of Credit Facility, Commitment Fee Percentage	0.20%
Payments of Debt Issuance Costs	$ 3,964	0	$ 7,400
Prepayment % of annual excess cash flow	50.00%
Prepayment % for asset sales and casualty events	100.00%
Line of Credit Facility, Expiration Date	Nov. 06, 2023
Letters of Credit Outstanding, Amount	$ 200
Debt Instrument, Covenant Description	If at any time, commencing with the fiscal quarter ending March 31, 2019, the Company has revolving borrowings, unreimbursed letter of credit drawings and undrawn letters of credit outstanding in an amount in excess of 35.0% of the commitment amount under the New Revolving Facility, the Credit Agreement requires the Company to maintain a secured net leverage ratio of at least 3.25 to 1.0.
Debt Instrument, Covenant Compliance	The Company is in compliance with all of the above covenants
Senior secured term loan facility due 2025
Debt Instrument
Long-term Debt, Gross	$ 145,000	$ 396,000	400,000
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	2.15%
Payments of Debt Issuance Costs			$ 5,100
Debt Instrument, Maturity Date	Nov. 06, 2025