Annual report pursuant to Section 13 and 15(d)

Fair Value of Financial Instruments (Details)

v3.3.1.900
Fair Value of Financial Instruments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Aug. 18, 2014
Apr. 23, 2014
Dec. 21, 2012
Oct. 17, 2012
Debt Instrument [Line Items]              
Asset retirement obligation capitalized $ 8,800 $ 9,295 $ 3,556        
Unamortized discount 2,500            
Debt Instrument, Unamortized Discount (Premium), Net [1] (12,493) $ (14,658)          
Fair value of notes 846,900            
December Notes              
Debt Instrument [Line Items]              
Unamortized premium 300            
Debt Instrument, Unamortized Discount (Premium), Net 1,100            
August Notes              
Debt Instrument [Line Items]              
Unamortized premium 14,700            
Debt Instrument, Unamortized Discount (Premium), Net 4,900            
October Notes              
Debt Instrument [Line Items]              
Debt Instrument, Unamortized Discount (Premium), Net 5,100            
April Notes              
Debt Instrument [Line Items]              
Debt Instrument, Unamortized Discount (Premium), Net         $ 6,800    
Senior Notes              
Debt Instrument [Line Items]              
Carrying value of debt 944,600            
Unamortized discount             $ 3,700
Unamortized premium       $ 18,000   $ 500  
Level 3              
Debt Instrument [Line Items]              
Asset retirement obligation capitalized $ 8,800            
[1] The October Notes were issued at a price of 98.534% resulting in a gross discount of $3.7 million and an effective rate of 8.000%. The December Notes were issued at a price of 101.000% resulting in a gross premium of $0.5 million and an effective rate of 7.531%. The August Notes were issued at a price of 106.000% resulting in a gross premium of $18.0 million and an effective rate of 6.561%. The April Notes were issued at par. The premium and discount are being amortized using the effective interest method.