Debt (Details) $ in Thousands	1 Months Ended				12 Months Ended
	Jul. 31, 2015 USD ($)	Apr. 30, 2015 USD ($)	Aug. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Debt
Outstanding debt principal					$ 925,000	$ 800,000
Unamortized issuance costs and discounts					(64,122)	(51,638)
Long-term debt					860,878	748,362
Loss on extinguishment of debt					165	2,898
Net deferred financing costs					7,986	2,846
Scheduled maturities of debt during the five year period and thereafter
2020					185,714
Thereafter					739,286
Parent company
Debt
Net deferred financing costs					7,986	2,846
Facility
Debt
Outstanding debt principal					400,000	500,000
Total commitment				$ 1,500,000
Loss on extinguishment of debt				2,900
Net deferred financing costs					37,500
Amount outstanding					400,000
Undrawn availability					$ 1,100,000
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
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					$ 8,000
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
Revolving Letter of Credit Facility
Debt
Total commitment	$ 75,000
Amount outstanding					$ 15,300
Number of letters of credit | item					9
Other disclosures
Cash collateral required as a percentage of outstanding letters of credit under breach of certain financial covenants	100.00%
Letter of credit fee (as a percent)	0.50%
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
Senior Notes
Debt
Outstanding debt principal					$ 525,000	$ 300,000
Senior Notes | 7.875% senior notes due 2021
Debt
Interest rate					7.875%
Senior notes offering face amount		$ 225,000	$ 300,000		$ 525,000
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%