Debt (Details) $ in Thousands	1 Months Ended				3 Months Ended
	Mar. 31, 2016 USD ($) item	Jun. 30, 2015 USD ($)	Apr. 30, 2015 USD ($)	Aug. 31, 2014 USD ($)	Mar. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Jul. 31, 2015 USD ($)	Mar. 31, 2014 USD ($)
Debt
Outstanding debt principal	$ 1,075,000				$ 1,075,000	$ 925,000
Unamortized issuance costs and discount	(61,404)				(61,404)	(64,122)
Long-term debt	1,013,596				1,013,596	860,878
Net deferred financing costs	7,302				7,302	7,986
Scheduled maturities of debt during the five year period and thereafter
2019	32,271				32,271
2020	348,123				348,123
Thereafter	694,606				694,606
Facility
Debt
Outstanding debt principal	550,000				550,000	400,000
Total commitment								$ 1,500,000
Reduction in borrowing capacity	73,500
Current borrowing capacity	1,427,000				1,427,000
Net deferred financing costs	35,700				35,700
Amount outstanding	550,000				550,000
Undrawn availability	876,500				876,500
Revolving-credit sublimit amount after March 31, 2018	500,000				$ 500,000
Availability period of revolving-credit sublimit					1 month
Amount outstanding under letters of credit	0				$ 0
Corporate Revolver
Debt
Total commitment		$ 400,000
Percentage of the margin used to calculate commitment fees		30.00%
Net deferred financing costs	7,300				7,300
Amount outstanding	0				0
Undrawn availability	400,000				400,000
Revolving Letter of Credit Facility
Debt
Total commitment							$ 75,000
Amount outstanding	$ 15,300				$ 15,300
Number of letters of credit | item	9				9
Revolving Letter of Credit Facility | Maximum
Debt
Additional commitments							$ 50,000
Senior Notes
Debt
Outstanding debt principal	$ 525,000				$ 525,000	$ 525,000
Senior Notes | 7.875% senior notes due 2021
Debt
Interest rate	7.875%				7.875%
Senior notes offering face amount			$ 225,000	$ 300,000
Proceeds, net of offering discounts and deferred financing costs			$ 206,800	$ 292,500