Investment Securities - Cost basis, unrealized gains and losses, and fair value of AFS debt securities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
AFS debt securities:
Amortized Cost	$ 4,516,529	$ 3,152,111
Gross Unrealized Gains	36,593	75,108
Gross Unrealized Losses	56,298	2,498
Fair Value	4,496,824	3,224,721
U.S. Treasuries
AFS debt securities:
Amortized Cost	218,027	123,677
Gross Unrealized Gains	1,661	4,395
Gross Unrealized Losses	2,168	0
Fair Value	217,520	128,072
U.S. Government agencies & GSEs
AFS debt securities:
Amortized Cost	189,855	152,596
Gross Unrealized Gains	605	701
Gross Unrealized Losses	3,428	325
Fair Value	187,032	152,972
State and political subdivisions
AFS debt securities:
Amortized Cost	263,269	253,630
Gross Unrealized Gains	15,237	20,891
Gross Unrealized Losses	2,662	49
Fair Value	275,844	274,472
Residential MBS, Agency & GSE
AFS debt securities:
Amortized Cost	2,079,700	1,275,551
Gross Unrealized Gains	9,785	29,107
Gross Unrealized Losses	28,521	766
Fair Value	2,060,964	1,303,892
Residential MBS, Non-agency
AFS debt securities:
Amortized Cost	81,925	174,322
Gross Unrealized Gains	2,249	7,499
Gross Unrealized Losses	4	128
Fair Value	84,170	181,693
Commercial MBS, Agency & GSE
AFS debt securities:
Amortized Cost	870,563	524,852
Gross Unrealized Gains	2,974	8,013
Gross Unrealized Losses	16,156	597
Fair Value	857,381	532,268
Commercial MBS, Non-agency
AFS debt securities:
Amortized Cost	15,202	15,350
Gross Unrealized Gains	1,268	1,513
Gross Unrealized Losses	0	0
Fair Value	16,470	16,863
Corporate bonds
AFS debt securities:
Amortized Cost	194,164	70,057
Gross Unrealized Gains	814	1,711
Gross Unrealized Losses	1,812	1
Fair Value	193,166	71,767
Asset-backed securities
AFS debt securities:
Amortized Cost	603,824	562,076
Gross Unrealized Gains	2,000	1,278
Gross Unrealized Losses	1,547	632
Fair Value	$ 604,277	$ 562,722

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://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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