Derivative Instruments Credit Rating (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Derivative [Line Items]
Unrealized losses on derivatives	$ (19,500,000)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	2,200,000	$ 2,900,000	$ 1,700,000
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	19,900,000	$ 23,300,000	$ 2,900,000
(Current Ratings) A/A3
Derivative [Line Items]
With Adequate Assurance Calls	0
Without Adequate Assurance Calls	0
BBB/Baa1
Derivative [Line Items]
With Adequate Assurance Calls	0
Without Adequate Assurance Calls	0
BBB/Baa2
Derivative [Line Items]
With Adequate Assurance Calls	0
Without Adequate Assurance Calls	0
BBB-/Baa3
Derivative [Line Items]
With Adequate Assurance Calls	(8,851,000)
Without Adequate Assurance Calls	(7,112,000)
Speculative
Derivative [Line Items]
With Adequate Assurance Calls	(30,172,000)
Without Adequate Assurance Calls	(18,433,000)
Financial [Member]
Derivative [Line Items]
Additional Collateral, Aggregate Fair Value	$ 0