Derivatives (Narrative) (Details) $ in Thousands		9 Months Ended
		Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Derivative [Line Items]
Notional amount of derivatives		$ 3,989,850		$ 3,024,565
Available for sale debt securities, Amortized Cost		3,480,949
Fair Values, Liabilities		14,620		2,160
Fair value liability		6,200		7,300
Other Liabilities [Member]
Derivative [Line Items]
Fair Values, Liabilities	[1]	88,555		44,815
Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Notional amount of derivatives		1,679,510		1,358,110
Derivatives Designated as Hedging Instruments [Member] | Other Liabilities [Member]
Derivative [Line Items]
Fair Values, Liabilities	[1]	7,952		$ 11,262
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member]
Derivative [Line Items]
Termination fees		10,600
Amortization of accumulated other comprehensive loss on terminated cash flow hedges		$ 3,400	$ 4,100
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement One To Five [Member]
Derivative [Line Items]
Number of interest rate swap agreements | item		5
Number of interest rate swap terminated | item		5
Notional amount of derivatives		$ 450,000	$ 450,000
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement 4, Expires 2021 [Member]
Derivative [Line Items]
Notional amount of derivatives		$ 50,000
Derivative maturity expiration year		2021
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement 1, Expires 2022 [Member]
Derivative [Line Items]
Notional amount of derivatives		$ 475,000
Derivative maturity expiration year		2022
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement 2, Expires 2023 [Member]
Derivative [Line Items]
Notional amount of derivatives		$ 450,000
Derivative maturity expiration year		2023
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member] | Swap Agreement 3, Expires 2024 [Member]
Derivative [Line Items]
Notional amount of derivatives		$ 100,000
Derivative maturity expiration year		2024
Interest Rate Swaps - Securities [Member] | Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedging [Member]
Derivative [Line Items]
Notional amount of derivatives		$ 441,400
Interest Rate Swaps - Securities [Member] | Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedging [Member] | Commercial Mortgage-Backed Securities [Member]
Derivative [Line Items]
Available for sale debt securities, Amortized Cost		499,200
Interest Rate Swaps - Securities [Member] | Derivatives Designated as Hedging Instruments [Member] | Fair Value Hedging [Member] | Other Liabilities [Member]
Derivative [Line Items]
Fair Values, Liabilities	[1]	$ 7,872

[1] Derivative assets and liabilities are reported at fair value in other assets or other liabilities, respectively, in the consolidated balance sheets.