Summary of Significant Accounting Policies - Provision for credit losses (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accounting Policies [Abstract]
Provision (credit) for credit losses- loans	$ (3,911)	$ 19,599	$ (6,172)	$ 33,909
Provision (credit) for credit losses- AFS debt securities	55	138	(34)	138
Total provision (credit) for credit losses	$ (3,856)	$ 19,737	$ (6,206)	$ 34,047

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Amount of expense (reversal of expense) for expected credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), from change in present value of expected cash flows.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82922868-210454

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
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-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922897-210455

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- Definition

Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 10
-Section 50
-Paragraph 11B
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
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-Section 45
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Amount of credit loss expense (reversal of expense) for financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
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