Derivatives and Hedging Activities - Schedule of Interest Rate Derivatives (Details) - Designated as Hedging Instrument - Cash Flow Hedging - Swap $ in Thousands	Jun. 30, 2018 USD ($) derivative	Dec. 31, 2017 USD ($) derivative
Derivative [Line Items]
Number of Instruments (derivative) | derivative	33	35
Notional Amount | $	$ 652,622	$ 673,345
GBP
Derivative [Line Items]
Number of Instruments (derivative) | derivative	17	19
Notional Amount | $	$ 285,920	$ 301,155
EUR
Derivative [Line Items]
Number of Instruments (derivative) | derivative	13	13
Notional Amount | $	$ 216,702	$ 222,190
USD
Derivative [Line Items]
Number of Instruments (derivative) | derivative	3	3
Notional Amount | $	$ 150,000	$ 150,000