Derivative Instruments and Hedging Activities Open Interest Rate Swap Contracts (Details) - Cash Flow Hedging - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Interest rate swap
Derivative [Line Items]
Notional Amounts	$ 525,000	$ 425,000
Net Unrealized Gain (Loss)	(8,648)	(2,665)
Interest Rate Swap, May 2018 to May 2021
Derivative [Line Items]
Notional Amounts	25,000	25,000
Net Unrealized Gain (Loss)	(45)	397
Interest Rate Swap, September 2018 to September 2019
Derivative [Line Items]
Notional Amounts	250,000	250,000
Net Unrealized Gain (Loss)	(259)	(163)
Interest Rate Swap, September 2019 to September 2023
Derivative [Line Items]
Notional Amounts	150,000	150,000
Net Unrealized Gain (Loss)	(7,947)	(2,899)
Interest Rate Swap, May 2019 To May 2020 [Member] [Member]
Derivative [Line Items]
Notional Amounts	100,000	0
Net Unrealized Gain (Loss)	$ (397)	$ 0