Derivative Instruments Credit Rating (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Derivative [Line Items]
Unrealized losses on derivatives	$ (14,300,000)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	2,500,000	$ 2,900,000	$ 900,000
Derivative liability	15,000,000	$ 23,300,000	7,400,000
(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	(3,605,000)
Without Adequate Assurance Calls	(3,605,000)
Speculative
Derivative [Line Items]
With Adequate Assurance Calls	(11,211,000)
Without Adequate Assurance Calls	(6,987,000)
Financial [Member]
Derivative [Line Items]
Additional Collateral, Aggregate Fair Value	$ 0		$ 0