Derivative Commodity Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Assets
Gross Amounts Recognized at Fair Value	$ 50
Gross Amounts Offset in the Balance Sheets	3,185
Total assets	3,235
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	3,235
Liabilities
Gross Amounts Recognized at Fair Value	56,142
Gross Amounts Offset in the Balance Sheets	3,185
Net Amount Presented in the Balance Sheets	59,327
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	59,327
Total Derivatives	(56,092)	$ 56,504
Current Assets
Assets
Gross Amounts Recognized at Fair Value	50	72,563
Gross Amounts Offset in the Balance Sheets	3,185	(15,600)
Total assets	3,235	56,963
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	3,235	56,963
Current Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	43,267	16,059
Gross Amounts Offset in the Balance Sheets	3,185	(15,600)
Net Amount Presented in the Balance Sheets	46,452	459
Financial Instruments	0	0
Cash Collateral Received	0	0
Net Fair Value Presented in the Balance Sheets	46,452	$ 459
Noncurrent Liabilities
Liabilities
Gross Amounts Recognized at Fair Value	12,875
Gross Amounts Offset in the Balance Sheets	0
Net Amount Presented in the Balance Sheets	12,875
Financial Instruments	0
Cash Collateral Received	0
Net Fair Value Presented in the Balance Sheets	$ 12,875