Derivative and Hedging Activities - Cumulative Basis Adjustment for Fair Value Hedges (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Balance Sheet Recording of Fair Value Hedge [Line Items]
Carrying Amount of the Hedged Assets/(Liabilities)	$ 132,919	$ 69,675
Designated as Hedging Instrument
Balance Sheet Recording of Fair Value Hedge [Line Items]
Carrying Amount of the Hedged Assets/(Liabilities)	500,000
Cumulative Amount of Fair Value Hedging Adjustment Included in the Carrying Amount of the Hedged Assets/(Liabilities)	2,400
Amortized cost basis of hedged asset	1,200,000
adjustment [Member] | Designated as Hedging Instrument
Balance Sheet Recording of Fair Value Hedge [Line Items]
Cumulative Amount of Fair Value Hedging Adjustment Included in the Carrying Amount of the Hedged Assets/(Liabilities)	(2,409)
Carrying Value [Member] | Designated as Hedging Instrument
Balance Sheet Recording of Fair Value Hedge [Line Items]
Carrying Amount of the Hedged Assets/(Liabilities)	$ 497,591