Bank segment - Balance Sheets Data (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Assets
Cash and cash equivalents	$ 667,129			$ 199,877	$ 175,280
Investment securities
Available-for-sale investment securities, at fair value	1,266,412			1,429,667
Held-to-maturity, at amortized cost (fair value of $1,052,221 and $1,150,971, respectively)	1,212,005			1,251,747
Held-to-maturity investment securities	1,052,221
Stock in Federal Home Loan Bank, at cost	18,000			26,560
Allowance for credit losses	(76,366)	$ (69,068)		(72,216)	(70,406)	$ (69,456)		$ (71,130)
Loans held for investment, net	6,114,640			5,906,690
Loans held for sale, at lower of cost or fair value	2,171			824
Other assets	826,020			824,536
Goodwill	82,190			82,190
Total assets	16,991,818			16,284,244
Liabilities and shareholder’s equity
Other	837,506			787,057
Total liabilities	14,733,171			14,047,452
Retained earnings	877,931			845,830
Accumulated other comprehensive loss, net of tax benefits
Accumulated other comprehensive loss, net of tax benefits	(358,024)			(336,028)
Common stock equity	2,224,354	$ 2,249,377	$ 2,237,955	2,202,499	$ 2,162,922	$ 2,233,504	$ 2,303,642	$ 2,390,884
Total liabilities and shareholders’ equity	16,991,818			16,284,244
Other assets
Premises and equipment, net	5,917,342			5,687,003
Other assets	826,020			824,536
Other liabilities
Other liabilities	837,506			787,057
American Savings Bank (ASB)
Assets
Cash and due from banks	139,059			153,042
Interest-bearing deposits	124,531			3,107
Cash and cash equivalents	263,590			156,149
Investment securities
Available-for-sale investment securities, at fair value	1,266,412			1,429,667
Held-to-maturity, at amortized cost (fair value of $1,052,221 and $1,150,971, respectively)	1,212,005			1,251,747
Held-to-maturity investment securities	1,052,221			1,150,971
Stock in Federal Home Loan Bank, at cost	18,000			26,560
Loans held for investment	6,191,006			5,978,906
Allowance for credit losses	(76,366)			(72,216)
Loans held for investment, net	6,114,640			5,906,690
Loans held for sale, at lower of cost or fair value	2,171			824
Other assets	698,420			692,143
Goodwill	82,190			82,190
Total assets	9,657,428			9,545,970
Liabilities and shareholder’s equity
Deposit liabilities—noninterest-bearing	2,573,010			2,811,077
Deposit liabilities—interest-bearing	5,651,341			5,358,619
Other borrowings	750,000			695,120
Other	224,136			212,269
Total liabilities	9,198,487			9,077,085
Common stock	1			1
Additional paid-in capital	357,742			355,806
Retained earnings	460,824			449,693
Accumulated other comprehensive loss, net of tax benefits
Net unrealized losses on securities	(350,234)			(328,904)
Retirement benefit plans	(9,392)			(7,711)
Accumulated other comprehensive loss, net of tax benefits	(359,626)			(336,615)
Common stock equity	458,941			468,885
Total liabilities and shareholders’ equity	9,657,428			9,545,970
Other assets
Bank-owned life insurance	186,143			182,986
Premises and equipment, net	189,950			195,324
Accrued interest receivable	29,361			25,077
Mortgage-servicing rights	8,376			9,047
Low-income housing investments	103,580			106,978
Deferred tax asset	127,735			116,441
Real estate acquired in settlement of loans, net	0			115
Other	53,275			56,175
Other assets	698,420			692,143
Other liabilities
Accrued expenses	102,540			97,295
Federal and state income taxes payable	845			863
Cashier’s checks	38,483			36,401
Advance payments by borrowers	4,289			9,637
Other	77,979			68,073
Other liabilities	$ 224,136			$ 212,269