Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Assets
Cash and cash equivalents	$ 315,334	$ 199,877	$ 301,704
Investment securities
Available-for-sale investment securities, at fair value	1,419,755	1,429,667
Held-to-maturity, at amortized cost (fair value of $1,158,090 and $1,150,971, respectively)	1,238,185	1,251,747
Held-to-maturity investment securities	1,158,090
Stock in Federal Home Loan Bank, at cost	10,000	26,560
Allowance for credit losses	(71,296)	(72,216)	(67,211)	$ (71,130)
Total loans, net	5,988,058	5,906,690
Loans held for sale, at lower of cost or fair value	660	824
Other assets	812,836	824,536
Goodwill	82,190	82,190
Total assets	16,443,785	16,284,244
Liabilities and shareholder’s equity
Other	727,919	787,057
Total liabilities	14,171,537	14,047,452
Retained earnings	861,105	845,830
Accumulated other comprehensive loss, net of tax benefits
Accumulated other comprehensive loss, net of tax benefits	(315,540)	(336,028)
Total shareholders’ equity	2,237,955	2,202,499	$ 2,303,642	$ 2,390,884
Total liabilities and shareholders’ equity	16,443,785	16,284,244
Other assets
Premises and equipment, net	5,778,571	5,687,003
Other assets	812,836	824,536
Other liabilities
Other liabilities	727,919	787,057
American Savings Bank (ASB)
Assets
Cash and due from banks	138,742	153,042
Interest-bearing deposits	44,315	3,107
Cash and cash equivalents	183,057	156,149
Investment securities
Available-for-sale investment securities, at fair value	1,419,755	1,429,667
Held-to-maturity, at amortized cost (fair value of $1,158,090 and $1,150,971, respectively)	1,238,185	1,251,747
Held-to-maturity investment securities	1,158,090	1,150,971
Stock in Federal Home Loan Bank, at cost	10,000	26,560
Loans held for investment	6,059,354	5,978,906
Allowance for credit losses	(71,296)	(72,216)
Total loans, net	5,988,058	5,906,690
Loans held for sale, at lower of cost or fair value	660	824
Other assets	688,165	692,143
Goodwill	82,190	82,190
Total assets	9,610,070	9,545,970
Liabilities and shareholder’s equity
Deposit liabilities—noninterest-bearing	2,769,789	2,811,077
Deposit liabilities—interest-bearing	5,460,812	5,358,619
Other borrowings	680,690	695,120
Other	206,317	212,269
Total liabilities	9,117,608	9,077,085
Common stock	1	1
Additional paid-in capital	356,391	355,806
Retained earnings	454,255	449,693
Accumulated other comprehensive loss, net of tax benefits
Net unrealized losses on securities	(308,622)	(328,904)
Retirement benefit plans	(9,563)	(7,711)
Accumulated other comprehensive loss, net of tax benefits	(318,185)	(336,615)
Total shareholders’ equity	492,462	468,885
Total liabilities and shareholders’ equity	9,610,070	9,545,970
Other assets
Bank-owned life insurance	183,936	182,986
Premises and equipment, net	192,789	195,324
Accrued interest receivable	26,547	25,077
Mortgage-servicing rights	8,745	9,047
Low-income housing investments	110,748	106,978
Real estate held for sale	100	0
Deferred tax asset	109,885	116,441
Real estate acquired in settlement of loans, net	614	115
Other	54,801	56,175
Other assets	688,165	692,143
Other liabilities
Accrued expenses	95,066	97,295
Federal and state income taxes payable	2,614	863
Cashier’s checks	34,616	36,401
Advance payments by borrowers	5,466	9,637
Other	68,555	68,073
Other liabilities	$ 206,317	$ 212,269