Debt (Details) (USD $)	3 Months Ended	1 Months Ended	3 Months Ended	1 Months Ended
	Mar. 31, 2014	Mar. 31, 2014	Mar. 31, 2015	Apr. 30, 2015	Aug. 31, 2014	Dec. 31, 2014	Nov. 30, 2012	Jul. 31, 2013
Debt
Outstanding debt principal			$ 800,000,000			$ 800,000,000
Unamortized issuance discounts			5,566,000			5,731,000
Long-term debt			794,434,000			794,269,000
Loss on extinguishment of debt	2,898,000
Net deferred financing costs			46,143,000			48,753,000
Scheduled maturities of debt during the five year period and thereafter
Thereafter			800,000,000
Facility
Debt
Outstanding debt principal			500,000,000			500,000,000
Total commitment	1,500,000,000	1,500,000,000
Loss on extinguishment of debt		2,900,000
Net deferred financing costs			42,800,000
Amount outstanding			500,000,000
Undrawn availability			1,000,000,000
Revolving-credit sublimit amount after March 31, 2018			500,000,000
Availability period of revolving-credit sublimit			1 month
Amount outstanding under letters of credit			0
Corporate Revolver
Debt
Total commitment							300,000,000
Net deferred financing costs			2,100,000
Amount outstanding			0
Undrawn availability			300,000,000
Revolving Letter of Credit Facility
Debt
Total commitment								100,000,000
Amount outstanding			21,500,000
Number of letters of credit			7
Revolving Letter of Credit Facility | Maximum
Debt
Additional commitments								50,000,000
Senior Notes
Debt
Outstanding debt principal			300,000,000			300,000,000
Senior notes offering face amount				225,000,000
Proceeds, net of offering discounts and deferred financing costs				206,800,000
Senior Notes | 7.875% senior notes due 2021
Debt
Interest rate			7.875%
Senior notes offering face amount				225,000,000	300,000,000
Proceeds, net of offering discounts and deferred financing costs				$ 206,800,000	$ 292,500,000