Fair Value Measurements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total additional asset retirement obligations incurred for the period	$ 8,700	$ 7,700
Interest Rate Swap Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets associated with interest rate swap contracts	14,137
Liabilities associated with interest rate swap contracts	(30,989)	(45,199)
Interest Rate Swap Contracts | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets associated with interest rate swap contracts	14,137
Liabilities associated with interest rate swap contracts	(30,989)	$ (45,199)
Senior Notes 5.125 Percent Due 2022 [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long term notes	253,900
Convertible Senior Notes 1.00 percent due 2020 [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of long term notes	$ 338,200