Loans and Allowances for Credit Losses, Activity in Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Mar. 31, 2025	Mar. 31, 2024	Jun. 30, 2024	Dec. 31, 2024
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 280,035	$ 277,123	$ 277,123	$ 277,123
Provision for loan losses	(336)	9,960
Loans charged off	(2,291)	(7,060)
Recoveries of loans previously charged off	1,186	1,600
Ending balance	278,594	281,623		280,035
Provision Details [Abstract]
Provision for credit losses	0	8,000
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Beginning balance	51,640	48,977	48,977	48,977
Provision for off-balance sheet credit risk	448	(1,658)
Ending balance	52,088	47,319		51,640
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment	23,606,968			24,067,999
Individually measured for impairment, recorded investment	83,520			46,725
Loans	23,690,488			24,114,724
Related allowance [Abstract]
Collectively measured for impairment, related allowance	278,594			279,759
Individually measured for impairment, related allowance	0			276
Total	278,594	281,623		280,035
Commercial [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	145,153	141,232	141,232	141,232
Provision for loan losses	(855)	8,311
Loans charged off	(1,085)	(4,240)
Recoveries of loans previously charged off	292	964
Ending balance	143,505	146,267		145,153
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Beginning balance	18,046	19,762	19,762	19,762
Provision for off-balance sheet credit risk	(1,879)	(1,972)
Ending balance	16,167	17,790		18,046
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment	14,360,364			15,015,489
Individually measured for impairment, recorded investment	43,067			14,647
Loans	14,403,431			15,030,136
Related allowance [Abstract]
Collectively measured for impairment, related allowance	143,505			144,877
Individually measured for impairment, related allowance	0			276
Total	143,505	146,267		145,153
Commercial real estate [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	91,072	94,718	94,718	94,718
Provision for loan losses	2,467	3,995
Loans charged off	0	(1,250)	(1,455)
Recoveries of loans previously charged off	185	16
Ending balance	93,724	97,479		91,072
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Beginning balance	31,959	27,439	27,439	27,439
Provision for off-balance sheet credit risk	1,502	426
Ending balance	33,461	27,865		31,959
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment	5,150,411			5,048,547
Individually measured for impairment, recorded investment	13,125			9,905
Loans	5,163,536			5,058,452
Related allowance [Abstract]
Collectively measured for impairment, related allowance	93,724			91,072
Individually measured for impairment, related allowance	0			0
Total	93,724	97,479		91,072
Loans to individuals [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	43,810	41,173	41,173	41,173
Provision for loan losses	(1,948)	(2,346)
Loans charged off	(1,206)	(1,570)
Recoveries of loans previously charged off	709	620
Ending balance	41,365	37,877		43,810
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Beginning balance	1,635	1,776	$ 1,776	1,776
Provision for off-balance sheet credit risk	825	(112)
Ending balance	2,460	1,664		1,635
Recorded investment [Abstract]
Collectively measured for impairment, recorded investment	4,096,193			4,003,963
Individually measured for impairment, recorded investment	27,328			22,173
Loans	4,123,521			4,026,136
Related allowance [Abstract]
Collectively measured for impairment, related allowance	41,365			43,810
Individually measured for impairment, related allowance	0			0
Total	$ 41,365	$ 37,877		$ 43,810

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Amount of allowance for credit loss on financing receivable evaluated on collective basis. Excludes financing receivable evaluated on individual basis.

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Amount of expected credit loss for credit exposure on off-balance-sheet commitment, including but not limited to, loan commitment, standby letter of credit, financial guarantee not accounted for as insurance. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

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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.

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