Derivatives - Effect on Income (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Gain (loss) on derivative
Gains or losses that were excluded from the assessment of the effectiveness of fair value hedges		$ 0	$ 0
Revenue
Gain (loss) on derivative
Net (losses) gains related to the ineffectiveness between the entity's derivative hedging instruments and hedged items		(100)
Cost of revenue.
Gain (loss) on derivative
Net (losses) gains related to the ineffectiveness between the entity's derivative hedging instruments and hedged items		(2,900)
Commodity contracts
Derivative, credit-risk related contingent features
Net liability position		50,000		$ 63,200
Collateral posted		(11,300)		(28,100)
Contingent additional collateral		38,700		35,000
Cash flow hedges.
Gain (loss) on derivative
Amount of Loss Recognized in Accumulated Other Comprehensive Income (Effective Portion)	[1]	(2,200)
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Income (Effective Portion)	[1]	500
Amount of Gain Recognized in Income (Ineffective Portion)	[1]	2,800
Cash flow hedges. | Commodity contracts | Revenue
Gain (loss) on derivative
Amount of Loss Recognized in Accumulated Other Comprehensive Income (Effective Portion)	[1]	(46,500)
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Income (Effective Portion)	[1]	(53,800)
Amount of Loss Recognized in Income (Ineffective Portion)	[1]	(100)
Cash flow hedges. | Commodity contracts | Cost of revenue.
Gain (loss) on derivative
Amount of Gain Recognized in Accumulated Other Comprehensive Income (Effective Portion)	[1]	44,200
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Income (Effective Portion)	[1]	54,300
Amount of Gain Recognized in Income (Ineffective Portion)	[1]	2,900
Derivatives designated as hedging instruments | Fair value hedges
Gain (loss) on derivative
Realized and Unrealized Gain (Loss) on Hedge	[1]	6,100	4,000
Realized and Unrealized Gain (Loss) on Hedged Item	[1]	(3,200)	(500)
Derivatives designated as hedging instruments | Fair value hedges | Commodity contracts | Revenue
Gain (loss) on derivative
Realized and Unrealized Gain (Loss) on Hedge	[1]	88,000
Derivatives designated as hedging instruments | Fair value hedges | Commodity contracts | Cost of revenue.
Gain (loss) on derivative
Realized and Unrealized Gain (Loss) on Hedge	[1]	(81,900)		4,000
Realized and Unrealized Gain (Loss) on Hedged Item	[1]	(3,200)		$ (500)
Derivatives not designated as hedging instruments
Gain (loss) on derivative
Realized and Unrealized Gain (Loss)	[1]	(11,900)	22,000
Derivatives not designated as hedging instruments | Commodity contracts | Revenue
Gain (loss) on derivative
Realized and Unrealized Gain (Loss)	[1]	38,900	20,600
Derivatives not designated as hedging instruments | Commodity contracts | Cost of revenue.
Gain (loss) on derivative
Realized and Unrealized Gain (Loss)	[1]	(47,500)	(11,700)
Derivatives not designated as hedging instruments | Foreign currency contracts | Revenue
Gain (loss) on derivative
Realized and Unrealized Gain (Loss)	[1]	1,900	3,900
Derivatives not designated as hedging instruments | Foreign currency contracts | Other income (expense), net
Gain (loss) on derivative
Realized and Unrealized Gain (Loss)	[1]	$ (5,200)	$ 9,200

[1] Amounts may not add due to rounding