Derivative Instruments and Hedging Activities Open Interest Rate Swap Contracts (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Net Unrealized Gain (Loss)	$ 3,789	$ (426)
Cash Flow Hedging | Interest rate swap
Derivative [Line Items]
Notional Amounts	425,000
Net Unrealized Gain (Loss)	779
Cash Flow Hedging | Interest Rate Swap, May 2018 to May 2021
Derivative [Line Items]
Notional Amounts	25,000
Net Unrealized Gain (Loss)	682
Cash Flow Hedging | Interest Rate Swap, September 2018 to September 2019
Derivative [Line Items]
Notional Amounts	250,000
Net Unrealized Gain (Loss)	(65)
Cash Flow Hedging | Interest Rate Swap, September 2019 to September 2023
Derivative [Line Items]
Notional Amounts	150,000
Net Unrealized Gain (Loss)	$ 162