Derivative Instruments - Credit Rating (Details) $ in Thousands	Sep. 30, 2017 USD ($)
(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	(3,138)
Without Adequate Assurance Calls	(3,138)
Speculative
Derivative [Line Items]
With Adequate Assurance Calls	(9,146)
Without Adequate Assurance Calls	$ (7,113)