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Fair Value Measurements (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Acquisition-related contingent consideration      
Liabilities associated with acquisition-related contingent consideration $ (43,175)    
Additions to asset retirement obligations liability      
Total additional asset retirement obligations incurred for the period 6,630    
Level 3 [Member]      
Acquisition-related contingent consideration      
Liabilities associated with acquisition-related contingent consideration (43,175)    
Additions to asset retirement obligations liability      
Total additional asset retirement obligations incurred for the period 6,600 $ 4,800  
Interest Rate Swap Contracts [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets associated with interest rate swap contracts 12,919   $ 14,137
Liabilities associated with interest rate swap contracts (24,234)   (30,989)
Interest Rate Swap Contracts [Member] | Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets associated with interest rate swap contracts 12,919   14,137
Liabilities associated with interest rate swap contracts (24,234)   $ (30,989)
Senior Notes 5.125 Percent Due 2022 | Level 2 [Member]      
Long-term debt      
Fair value of long term notes 256,300    
Convertible Senior Notes 1.00 percent due 2020 | Level 2 [Member]      
Long-term debt      
Fair value of long term notes 275,300    
Senior Notes 5.50 Percent Due 2025 | Level 2 [Member]      
Long-term debt      
Fair value of long term notes $ 309,300