Derivatives (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Dec. 31, 2020
Derivative [Line Items]
Notional amount of derivatives	$ 8,622,988	$ 7,975,022
Fair value liability	5,300	5,600
Credit risk-related contingent features, net liability position	16,000	109,700
Credit risk-related contingent features, posted collateral	18,400	44,700
Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Notional amount of derivatives	2,945,150	2,333,150
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 5, Expires Thereafter [Member]
Derivative [Line Items]
Notional amount of derivatives	$ 250,000
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	$ 1,520,150	$ 1,158,150
Derivative hedged item	1,400,000
Basis adjustment associated with hedged items	55,500
Interest Rate Swaps - Securities [Member] \| Derivatives Designated as Hedging Instruments [Member] \| Fair Value Hedging [Member] \| Commercial Mortgage-Backed Securities [Member]
Derivative [Line Items]
Amortized cost basis of closed portfolio of pre-payable securities	$ 1,700,000

Derivative, Maturity Year

No definition available.

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- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624258-113959

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- Definition

Amount of hedged item in hedging relationship. Excludes hedged asset or liability.

+ References

No definition available.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624258-113959

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- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-Section 50
-Paragraph 1A
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- Definition

Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk, in last-of-layer hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EEE
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109999718-113959

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

Amortized cost of closed portfolio of prepayable financial asset, used to designate hedging relationship in which hedged item is last layer expected to be remaining at end of hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EEE
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