Investments - Cost basis, unrealized gains and losses, and fair value of AFS debt securities (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
AFS debt securities:
Amortized Cost	$ 3,971,326	$ 4,516,529
Gross Unrealized Gains	1,132	36,593
Gross Unrealized Losses	358,125	56,298
Fair Value	3,614,333	4,496,824
U.S. Treasuries
AFS debt securities:
Amortized Cost	163,972	218,027
Gross Unrealized Gains	0	1,661
Gross Unrealized Losses	14,620	2,168
Fair Value	149,352	217,520
U.S. Government agencies & GSEs
AFS debt securities:
Amortized Cost	266,347	189,855
Gross Unrealized Gains	463	605
Gross Unrealized Losses	16,694	3,428
Fair Value	250,116	187,032
State and political subdivisions
AFS debt securities:
Amortized Cost	329,723	263,269
Gross Unrealized Gains	151	15,237
Gross Unrealized Losses	26,126	2,662
Fair Value	303,748	275,844
Residential MBS, Agency & GSE
AFS debt securities:
Amortized Cost	1,609,442	2,079,700
Gross Unrealized Gains	13	9,785
Gross Unrealized Losses	160,636	28,521
Fair Value	1,448,819	2,060,964
Residential MBS, Non-agency
AFS debt securities:
Amortized Cost	374,535	81,925
Gross Unrealized Gains	0	2,249
Gross Unrealized Losses	27,873	4
Fair Value	346,662	84,170
Commercial MBS, Agency & GSE
AFS debt securities:
Amortized Cost	720,282	870,563
Gross Unrealized Gains	471	2,974
Gross Unrealized Losses	79,407	16,156
Fair Value	641,346	857,381
Commercial MBS, Non-agency
AFS debt securities:
Amortized Cost	31,624	15,202
Gross Unrealized Gains	0	1,268
Gross Unrealized Losses	1,058	0
Fair Value	30,566	16,470
Corporate bonds
AFS debt securities:
Amortized Cost	236,181	194,164
Gross Unrealized Gains	34	814
Gross Unrealized Losses	23,763	1,812
Fair Value	212,452	193,166
Asset-backed securities
AFS debt securities:
Amortized Cost	239,220	603,824
Gross Unrealized Gains	0	2,000
Gross Unrealized Losses	7,948	1,547
Fair Value	$ 231,272	$ 604,277

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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-Paragraph 2
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Amortized cost excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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