Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets
Gross Amounts Recognized at Fair Value	$ 8,478
Gross Amounts Offset in the Balance Sheets	(4,825)
Net Amount Presented in the Balance Sheets	3,653
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	3,653
Liabilities
Gross Amounts Recognized at Fair Value	35,777
Gross Amounts Offset in the Balance Sheets	(4,825)
Net Amount Presented in the Balance Sheets	30,952
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	30,952
Total Derivatives	(27,299)	$ 56,504
Current Assets
Assets
Gross Amounts Recognized at Fair Value	8,308	72,563
Gross Amounts Offset in the Balance Sheets	(4,655)	(15,600)
Net Amount Presented in the Balance Sheets	3,653	56,963
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	3,653	56,963
Noncurrent Assets
Assets
Gross Amounts Recognized at Fair Value	170
Gross Amounts Offset in the Balance Sheets	(170)
Net Amount Presented in the Balance Sheets	0
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	0
Current Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	29,564	16,059
Gross Amounts Offset in the Balance Sheets	(4,655)	(15,600)
Net Amount Presented in the Balance Sheets	24,909	459
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	24,909	$ 459
Noncurrent Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	6,213
Gross Amounts Offset in the Balance Sheets	(170)
Net Amount Presented in the Balance Sheets	6,043
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	$ 6,043