Derivative Instruments - Credit Rating (Details) $ in Thousands	Jun. 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	(2,150)
Without Adequate Assurance Calls	(2,150)
Speculative
Derivative [Line Items]
With Adequate Assurance Calls	(4,829)
Without Adequate Assurance Calls	$ (3,576)