Derivative Instruments - Non-hedge Derivatives (Details) - Non-hedge derivatives - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Amount	$ 1,381,748	$ 1,333,144
Estimated Fair Value	(121)	(134)
Pay fixed / receive variable swaps
Derivative [Line Items]
Notional Amount	690,874	666,572
Estimated Fair Value	16,218	16,004
Pay variable / receive fixed swaps
Derivative [Line Items]
Notional Amount	690,874	666,572
Estimated Fair Value	$ (16,339)	$ (16,138)