Bank segment - Balance Sheets Data (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Assets
Cash and cash equivalents	$ 485,192	$ 679,546	$ 315,334
Investment securities
Available-for-sale investment securities, at fair value	1,091,889	1,136,439
Held-to-maturity, at amortized cost (fair value of $1,074,735 and $1,103,668, at March 31, 2024 and December 31, 2023, respectively)	1,191,074	1,201,314
Held-to-maturity investment securities	1,074,735
Stock in Federal Home Loan Bank, at cost	32,489	14,728
Allowance for credit losses	(71,057)	(74,372)	(71,296)	$ (72,216)
Loans held for investment, net	6,045,665	6,106,438
Loans held for sale, at lower of cost or fair value	2,923	15,168
Other assets	915,205	877,959
Goodwill	82,190	82,190
Total assets	16,923,208	17,243,821
Liabilities and shareholder’s equity
Other	772,060	853,349
Total liabilities	14,511,535	14,864,687
Retained earnings	968,842	926,720
Accumulated other comprehensive loss, net of tax benefits
Accumulated other comprehensive loss, net of tax benefits	(299,151)	(289,350)
Common stock equity	2,377,380	2,344,841	$ 2,237,955	$ 2,202,499
Total liabilities and shareholders’ equity	16,923,208	17,243,821
Other assets
Premises and equipment, net	6,183,503	6,150,126
Other assets	915,205	877,959
Other liabilities
Other liabilities	772,060	853,349
American Savings Bank (ASB)
Assets
Cash and due from banks	126,259	184,383
Interest-bearing deposits	100,681	251,072
Cash and cash equivalents	226,940	435,455
Investment securities
Available-for-sale investment securities, at fair value	1,091,889	1,136,439
Held-to-maturity, at amortized cost (fair value of $1,074,735 and $1,103,668, at March 31, 2024 and December 31, 2023, respectively)	1,191,074	1,201,314
Held-to-maturity investment securities	1,074,735	1,103,668
Stock in Federal Home Loan Bank, at cost	32,489	14,728
Loans held for investment	6,116,722	6,180,810
Allowance for credit losses	(71,057)	(74,372)
Loans held for investment, net	6,045,665	6,106,438
Loans held for sale, at lower of cost or fair value	2,923	15,168
Other assets	687,059	681,460
Goodwill	82,190	82,190
Total assets	9,360,229	9,673,192
Liabilities and shareholder’s equity
Deposit liabilities—noninterest-bearing	2,557,240	2,599,762
Deposit liabilities—interest-bearing	5,447,824	5,546,016
Other borrowings	593,000	750,000
Other	220,570	247,563
Total liabilities	8,818,634	9,143,341
Common stock	1	1
Additional paid-in capital	358,645	358,067
Retained earnings	484,989	464,055
Accumulated other comprehensive loss, net of tax benefits
Net unrealized losses on securities	(293,466)	(282,963)
Retirement benefit plans	(8,574)	(9,309)
Accumulated other comprehensive loss, net of tax benefits	(302,040)	(292,272)
Common stock equity	541,595	529,851
Total liabilities and shareholders’ equity	9,360,229	9,673,192
Other assets
Bank-owned life insurance	191,448	187,857
Premises and equipment, net	184,684	187,042
Accrued interest receivable	29,697	28,472
Mortgage-servicing rights	8,050	8,169
Low-income housing investments	108,314	112,234
Deferred tax asset	108,240	104,292
Other	56,626	53,394
Other assets	687,059	681,460
Other liabilities
Accrued expenses	114,289	115,231
Cashier’s checks	34,908	40,479
Advance payments by borrowers	5,559	10,107
Other	65,814	81,746
Other liabilities	$ 220,570	$ 247,563