Debt (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Credit facility, total borrowing capacity	$ 470,300
Letters of Credit Outstanding, Amount	46,600
Revolving credit facility outstanding, amount	325,000
Long-term Debt, Gross	1,627,387	$ 1,608,089
Long-term Debt, Current Maturities	6,028	29,957
Unamortized debt issuance costs	18,812	19,826
Long-term Debt, Excluding Current Maturities, Total	1,608,575	1,588,263
Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, revolving borrowing capacity	600,000
Aggregate principal amount of term loan	325,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	325,000
Unsecured Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Senior Notes	$ 500,000
Stated interest rate on unsecured senior notes	6.375%
Long-term Debt, Gross		500,000
Unsecured Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Senior Notes	$ 800,000
Long-term Debt, Gross	800,000	800,000
Other Debt Obligations [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	2,387	3,089
Long-term Debt, Current Maturities	$ 6,028	9,957
Term loan under credit facility [Member]
Debt Instrument [Line Items]
Credit Facility, Term Loan Amount Outstanding		305,000
Long-term Debt, Current Maturities		$ 20,000