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Debt (Details) $ in Thousands	1 Months Ended						12 Months Ended
Debt (Details) $ in Thousands	Jul. 31, 2016 USD ($)	Jun. 30, 2016	Jul. 31, 2015 USD ($)	Apr. 30, 2015 USD ($)	Aug. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Feb. 28, 2017 USD ($)	Oct. 01, 2016 USD ($)	Aug. 01, 2015 USD ($)	Feb. 01, 2015 USD ($)
Debt
Outstanding debt principal							$ 1,375,000	$ 925,000
Unamortized issuance costs and discount							(53,126)	(64,122)
Long-term debt							1,321,874	860,878
Loss on extinguishment of debt								165	$ 2,898
Net deferred financing costs							5,248	7,986
Scheduled maturities of debt during the five year period and thereafter
2019							268,823
2020							395,166
2021							711,011
Parent company
Debt
Net deferred financing costs							5,248	7,986
Facility
Debt
Outstanding debt principal							850,000	400,000
Unamortized issuance costs and discount							30,300	37,500
Total commitment						$ 1,500,000
Current borrowing capacity											$ 1,467,000
Loss on extinguishment of debt						2,900
Net deferred financing costs							30,300
Amount outstanding							850,000
Undrawn availability							$ 616,900
Variable rate basis							LIBOR
Commitment fee percentage of the then-applicable margin when commitment is available for utilization							40.00%
Commitment fee percentage of the then-applicable margin when commitment is not available for utilization							20.00%
Adjustment in estimate of deferred interest						4,500
Revolving-credit sublimit amount after March 31, 2018							$ 500,000
Availability period of revolving-credit sublimit							1 month
Amount outstanding under letters of credit							$ 0
Interval period for payment of interest							6 months
Size of LC Facility						$ 1,500,000
Facility \| Minimum \| LIBOR
Debt
Applicable margin (as a percent)							3.25%
Facility \| Maximum \| LIBOR
Debt
Applicable margin (as a percent)							4.50%
Corporate Revolver
Debt
Total commitment							$ 400,000
Percentage of the margin used to calculate commitment fees							30.00%
Net deferred financing costs							$ 5,200
Amount outstanding							0
Undrawn availability							$ 400,000
Applicable margin (as a percent)							6.00%
Variable rate basis							LIBOR
Interval period for payment of interest							6 months
Size of LC Facility							$ 400,000
Revolving Letter of Credit Facility
Debt
Total commitment	$ 75,000		$ 75,000
Amount outstanding							$ 72,800
Number of letters of credit \| item							9
Applicable margin (as a percent)	0.80%	0.50%
Commitment fee payable	0.65%
Size of LC Facility	$ 75,000		$ 75,000
Other disclosures
Cash collateral required as a percentage of outstanding letters of credit under breach of certain financial covenants			100.00%
Revolving Letter of Credit Facility \| Forecast
Debt
Total commitment										$ 125,000
Size of LC Facility										$ 125,000
Revolving Letter of Credit Facility \| Minimum
Other disclosures
Cash collateral maintained as a percentage of outstanding letters of credit			75.00%
Revolving Letter of Credit Facility \| Maximum
Debt
Additional commitments	$ 50,000		$ 50,000
Senior Notes
Debt
Outstanding debt principal							$ 525,000	525,000
Unamortized issuance costs and discount							22,800	$ 26,600
Senior Notes \| 7.875% senior notes due 2021
Debt
Senior notes offering face amount				$ 225,000	$ 300,000		$ 525,000					$ 225,000	$ 300,000
Proceeds, net of offering discounts and deferred financing costs				$ 206,800	$ 292,500
Other disclosures
Redemption price percentage following change of control							101.00%
Redemption price percentage following sell of certain assets							100.00%
Senior Notes \| 7.875% senior notes due 2021 \| Prior to August 1, 2017
Other disclosures
Senior notes redemption, start date							Aug. 01, 2014
Senior notes redemption, end date							Aug. 01, 2017
Maximum percentage of principal amount available to be redeemed with proceeds from equity offerings							35.00%
Redemption price percentage using proceeds from equity offerings							107.875%
Redemption price percentage, excluding proceeds from equity offerings							100.00%
Senior Notes \| 7.875% senior notes due 2021 \| On or after August 1, 2017, but before August 1, 2018
Other disclosures
Redemption price, as a percent of the of principal amount							103.90%
Senior Notes \| 7.875% senior notes due 2021 \| On or after August 1, 2018, but before August 1, 2019
Other disclosures
Redemption price, as a percent of the of principal amount							102.00%
Senior Notes \| 7.875% senior notes due 2021 \| On or after August 1, 2019 and thereafter
Other disclosures
Redemption price, as a percent of the of principal amount							100.00%