Derivatives (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Derivative [Line Items]
Notional amount of derivatives		$ 6,822,497		$ 6,010,689
Available for sale debt securities, Amortized Cost		4,709,075
Fair Values, Liabilities		38,311		11,953
Fair value liability		5,300		5,700
Credit risk-related contingent features, net liability position		40,000		12,900
Credit risk-related contingent features, posted collateral		40,000		12,400
Other Liabilities [Member]
Derivative [Line Items]
Fair Values, Liabilities	[1]	199,244		65,003
Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Notional amount of derivatives		1,654,900		1,659,400
Derivatives Designated as Hedging Instruments [Member] | Other Liabilities [Member]
Derivative [Line Items]
Fair Values, Liabilities	[1]	23,453		2,105
Interest Rate Swaps [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedge [Member]
Derivative [Line Items]
Amortization of accumulated other comprehensive loss on terminated cash flow hedges		700	$ 1,400
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		$ 550,000
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		$ 479,900		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		473,200
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]	$ 23,453		$ 1,759

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