Derivative Accounting - Derivative Instruments in the Balance Sheets (Details) - Commodity Contracts - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Assets
Gross Recognized Derivatives	$ 29,040,000	$ 53,356,000
Amounts Offset	(22,962,000)	(26,017,000)
Net Recognized Derivatives	6,078,000	27,339,000
Other	157,000	672,000
Amount Reported on Balance Sheet	6,235,000	28,011,000
Liabilities
Gross Recognized Derivatives	(122,075,000)	(207,387,000)
Amounts Offset	22,962,000	44,077,000
Net Recognized Derivatives	(99,113,000)	(163,310,000)
Other	(4,405,000)	(4,379,000)
Amount Reported on Balance Sheet	(103,518,000)	(167,689,000)
Assets and Liabilities
Gross Recognized Derivatives	(93,035,000)	(154,031,000)
Amounts Offset	0	18,060,000
Net Recognized Derivatives	(93,035,000)	(135,971,000)
Other	(4,248,000)	(3,707,000)
Amount Reported on Balance Sheet	(97,283,000)	(139,678,000)
Current Assets
Assets
Gross Recognized Derivatives	24,894,000	37,396,000
Amounts Offset	(18,816,000)	(22,163,000)
Net Recognized Derivatives	6,078,000	15,233,000
Other	157,000	672,000
Amount Reported on Balance Sheet	6,235,000	15,905,000
Investments and Other Assets
Assets
Gross Recognized Derivatives	4,146,000	15,960,000
Amounts Offset	(4,146,000)	(3,854,000)
Net Recognized Derivatives	0	12,106,000
Other	0	0
Amount Reported on Balance Sheet	0	12,106,000
Current Liabilities
Liabilities
Gross Recognized Derivatives	(59,586,000)	(113,560,000)
Amounts Offset	18,816,000	40,223,000
Net Recognized Derivatives	(40,770,000)	(73,337,000)
Other	(4,405,000)	(4,379,000)
Amount Reported on Balance Sheet	(45,175,000)	(77,716,000)
Deferred Credits and Other
Liabilities
Gross Recognized Derivatives	(62,489,000)	(93,827,000)
Amounts Offset	4,146,000	3,854,000
Net Recognized Derivatives	(58,343,000)	(89,973,000)
Other	0	0
Amount Reported on Balance Sheet	$ (58,343,000)	$ (89,973,000)