Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets
Gross Amounts Recognized at Fair Value	$ 23,297
Gross Amounts Offset in the Balance Sheets	(4,339)
Total assets	18,958
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	18,958
Liabilities
Gross Amounts Recognized at Fair Value	10,080
Gross Amounts Offset in the Balance Sheets	(4,339)
Net Amount Presented in the Balance Sheets	5,741
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	5,741
Total Derivatives	13,217	$ 56,504
Current Assets
Assets
Gross Amounts Recognized at Fair Value	23,182	72,563
Gross Amounts Offset in the Balance Sheets	(4,372)	(15,600)
Total assets	18,810	56,963
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	18,810	56,963
Noncurrent Assets
Assets
Gross Amounts Recognized at Fair Value	115
Gross Amounts Offset in the Balance Sheets	33
Total assets	148
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	148
Current Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	9,840	16,059
Gross Amounts Offset in the Balance Sheets	(4,372)	(15,600)
Net Amount Presented in the Balance Sheets	5,468	459
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	5,468	$ 459
Noncurrent Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	240
Gross Amounts Offset in the Balance Sheets	33
Net Amount Presented in the Balance Sheets	273
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	$ 273