Bank segment - Balance Sheets Data (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Assets
Cash and cash equivalents	$ 314,284		$ 199,877	$ 159,672
Investment securities
Available-for-sale investment securities, at fair value	1,368,037		1,429,667
Held-to-maturity, at amortized cost (fair value of $1,121,995 and $1,150,971, respectively)	1,224,917		1,251,747
Held-to-maturity investment securities	1,121,995
Stock in Federal Home Loan Bank, at cost	18,000		26,560
Allowance for credit losses	(69,068)	$ (71,296)	(72,216)	(69,456)	$ (67,211)	$ (71,130)
Loans held for investment, net	6,069,114		5,906,690
Loans held for sale, at lower of cost or fair value	6,910		824
Other assets	795,214		824,536
Goodwill	82,190		82,190
Total assets	16,519,309		16,284,244
Liabilities and shareholder’s equity
Other	747,179		787,057
Total liabilities	14,235,639		14,047,452
Retained earnings	876,268		845,830
Accumulated other comprehensive loss, net of tax benefits
Accumulated other comprehensive loss, net of tax benefits	(323,149)		(336,028)
Common stock equity	2,249,377	$ 2,237,955	2,202,499	$ 2,233,504	$ 2,303,642	$ 2,390,884
Total liabilities and shareholders’ equity	16,519,309		16,284,244
Other assets
Premises and equipment, net	5,868,408		5,687,003
Other assets	795,214		824,536
Other liabilities
Other liabilities	747,179		787,057
American Savings Bank (ASB)
Assets
Cash and due from banks	158,170		153,042
Interest-bearing deposits	9,958		3,107
Cash and cash equivalents	168,128		156,149
Investment securities
Available-for-sale investment securities, at fair value	1,368,037		1,429,667
Held-to-maturity, at amortized cost (fair value of $1,121,995 and $1,150,971, respectively)	1,224,917		1,251,747
Held-to-maturity investment securities	1,121,995		1,150,971
Stock in Federal Home Loan Bank, at cost	18,000		26,560
Loans held for investment	6,138,182		5,978,906
Allowance for credit losses	(69,068)		(72,216)
Loans held for investment, net	6,069,114		5,906,690
Loans held for sale, at lower of cost or fair value	6,910		824
Other assets	683,395		692,143
Goodwill	82,190		82,190
Total assets	9,620,691		9,545,970
Liabilities and shareholder’s equity
Deposit liabilities—noninterest-bearing	2,683,725		2,811,077
Deposit liabilities—interest-bearing	5,479,510		5,358,619
Other borrowings	750,000		695,120
Other	212,268		212,269
Total liabilities	9,125,503		9,077,085
Common stock	1		1
Additional paid-in capital	357,123		355,806
Retained earnings	463,459		449,693
Accumulated other comprehensive loss, net of tax benefits
Net unrealized losses on securities	(315,917)		(328,904)
Retirement benefit plans	(9,478)		(7,711)
Accumulated other comprehensive loss, net of tax benefits	(325,395)		(336,615)
Common stock equity	495,188		468,885
Total liabilities and shareholders’ equity	9,620,691		9,545,970
Other assets
Bank-owned life insurance	183,833		182,986
Premises and equipment, net	192,070		195,324
Accrued interest receivable	27,136		25,077
Mortgage-servicing rights	8,495		9,047
Low-income housing investments	107,164		106,978
Deferred tax asset	112,345		116,441
Real estate acquired in settlement of loans, net	0		115
Other	52,352		56,175
Other assets	683,395		692,143
Other liabilities
Accrued expenses	98,664		97,295
Federal and state income taxes payable	187		863
Cashier’s checks	32,634		36,401
Advance payments by borrowers	10,800		9,637
Other	69,983		68,073
Other liabilities	$ 212,268		$ 212,269