Derivative Instruments- Interest Rate Swaptions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017
Derivative [Line Items]
Derivative assets	$ 20,571	$ 728
Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Derivative assets	$ 20,571	$ 728
Variable Pay Rate | Interest Rate Swaps 1.26%-1.50% | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Average Maturity (Years)	2 months 3 days
Variable Pay Rate | Interest rate swaption | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Average Maturity (Years)	2 months 3 days
Fixed Pay Rate | Interest Rate Swaption 1.76%-2.00% | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Average Maturity (Years)	2 months 3 days
Fixed Pay Rate | Interest Rate Swaption 2.01%-2.25% | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Average Maturity (Years)	2 months 3 days
Fixed Pay Rate | Interest Rate Swaption 2.26%-2.50% | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Average Maturity (Years)	5 months 24 days
Fixed Pay Rate | Interest rate swaption | Derivative instruments not accounted as hedges under GAAP
Derivative [Line Items]
Average Maturity (Years)	2 months 21 days