Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Assets
Cash and cash equivalents	$ 301,704	$ 305,551	$ 277,579
Investment securities
Available-for-sale investment securities, at fair value	2,621,375	2,574,618
Held-to-maturity, at amortized cost (fair value of $466,236 and $510,474, respectively)	517,150	522,270
Held-to-maturity investment securities	466,236
Stock in Federal Home Loan Bank, at cost	10,000	10,000
Allowance for credit losses	(67,211)	(71,130)	(91,793)	$ (101,201)
Total loans, net	5,117,522	5,139,984
Loans held for sale, at lower of cost or fair value	7,961	10,404
Other assets	777,694	747,469
Goodwill	82,190	82,190
Total assets	15,908,429	15,822,637
Liabilities and shareholder’s equity
Other	645,118	669,215
Total liabilities	13,570,494	13,397,460
Retained earnings	788,787	757,921
Accumulated other comprehensive loss, net of tax benefits
Accumulated other comprehensive loss, net of tax benefits	(169,692)	(52,533)
Total shareholders’ equity	2,303,642	2,390,884	$ 2,321,293	$ 2,337,502
Total liabilities and shareholders’ equity	15,908,429	15,822,637
Other assets
Premises and equipment, net	5,411,746	5,392,068
Other assets	777,694	747,469
Other liabilities
Other liabilities	645,118	669,215
American Savings Bank (ASB)
Assets
Cash and due from banks	114,249	100,051
Interest-bearing deposits	155,279	151,189
Cash and cash equivalents	269,528	251,240
Investment securities
Available-for-sale investment securities, at fair value	2,621,375	2,574,618
Held-to-maturity, at amortized cost (fair value of $466,236 and $510,474, respectively)	517,150	522,270
Held-to-maturity investment securities	466,236	510,474
Stock in Federal Home Loan Bank, at cost	10,000	10,000
Loans held for investment	5,184,733	5,211,114
Allowance for credit losses	(67,211)	(71,130)
Total loans, net	5,117,522	5,139,984
Loans held for sale, at lower of cost or fair value	7,961	10,404
Other assets	626,599	590,897
Goodwill	82,190	82,190
Total assets	9,252,325	9,181,603
Liabilities and shareholder’s equity
Deposit liabilities—noninterest-bearing	3,016,520	2,976,632
Deposit liabilities—interest-bearing	5,272,752	5,195,580
Other borrowings	137,385	88,305
Other	210,681	193,268
Total liabilities	8,637,338	8,453,785
Common stock	1	1
Additional paid-in capital	354,635	353,895
Retained earnings	420,574	411,704
Accumulated other comprehensive loss, net of tax benefits
Net unrealized losses on securities	(152,444)	(32,037)
Retirement benefit plans	(7,779)	(5,745)
Accumulated other comprehensive loss, net of tax benefits	(160,223)	(37,782)
Total shareholders’ equity	614,987	727,818
Total liabilities and shareholders’ equity	9,252,325	9,181,603
Other assets
Bank-owned life insurance	176,301	177,566
Premises and equipment, net	199,949	202,299
Accrued interest receivable	21,133	20,854
Mortgage-servicing rights	10,024	9,950
Low-income housing investments	107,791	110,989
Deferred tax asset	51,720	7,699
Other	59,681	61,540
Other assets	626,599	590,897
Other liabilities
Accrued expenses	104,958	87,905
Federal and state income taxes payable	2,174	0
Cashier’s checks	36,586	33,675
Advance payments by borrowers	5,664	9,994
Other	61,299	61,694
Other liabilities	$ 210,681	$ 193,268