Debt Securities Available for Sale and Debt Securities Held to Maturity - Allowance for credit losses for debt securities held to maturity (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2020
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period	$ 15,017	$ 0
Provision for credit losses	88	13,258
Securities charged off	0	0
Recoveries	0	0
Balance at end of period	$ 15,105	15,105
Impact of ASC 326 Adoption
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period		$ 1,847

Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aaa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

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Balance Type:	credit
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- Definition

Amount of increase in allowance for credit loss on investment in debt security measured at amortized cost (held-to-maturity), from recovery.

+ References

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

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- Definition

Amount of writeoff of investment in debt security, measured at amortized cost (held-to-maturity), charged against allowance for credit loss.

+ References

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- Definition

Amount of expense (reversal of expense) for expected credit loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

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