Derivative Instruments and Hedging Activities Open Interest Rate Swap Contracts (Details) - Cash Flow Hedging - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Interest rate swap
Derivative [Line Items]
Notional Amounts	$ 425,000	$ 425,000
Net Unrealized Gain (Loss)	(4,743)	(2,665)
Interest Rate Swap, May 2018 to May 2021
Derivative [Line Items]
Notional Amounts	25,000	25,000
Net Unrealized Gain (Loss)	235	397
Interest Rate Swap, September 2018 to September 2019
Derivative [Line Items]
Notional Amounts	250,000	250,000
Net Unrealized Gain (Loss)	(221)	(163)
Interest Rate Swap, September 2019 to September 2023
Derivative [Line Items]
Notional Amounts	150,000	150,000
Net Unrealized Gain (Loss)	$ (4,757)	$ (2,899)