Derivative Instruments Credit Rating (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Derivative [Line Items]
Unrealized losses on derivatives	$ 9,700,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	1,900,000	$ 2,900,000	$ 3,300,000
Derivative liability	10,100,000	$ 23,300,000	12,600,000
(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	(2,587,000)
Without Adequate Assurance Calls	(2,587,000)
Speculative
Derivative [Line Items]
With Adequate Assurance Calls	(7,023,000)
Without Adequate Assurance Calls	(4,730,000)
Financial [Member]
Derivative [Line Items]
Additional Collateral, Aggregate Fair Value	$ 0		$ 0