Bank segment - Balance sheet data (Details) - USD ($)	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Assets
Available-for-sale investment securities, at fair value	$ 1,747,658,000			$ 1,232,826,000
Held-to-maturity investment securities, at amortized cost	133,858,000			139,451,000
Stock in Federal Home Loan Bank, at cost	10,920,000			8,434,000
Net loans	5,389,443,000		$ 5,067,821,000	5,067,821,000
Loans held for sale, at lower of cost or fair value	16,806,000			12,286,000
Other	591,258,000			649,885,000
Goodwill	82,190,000			82,190,000
Total assets	14,542,361,000		13,745,251,000	13,745,251,000
Liabilities
Other	586,853,000		583,545,000	583,545,000
Total liabilities	12,184,837,000		11,430,698,000	11,430,698,000
Commitments and contingencies
Retained earnings	645,943,000		622,042,000	622,042,000
Accumulated other comprehensive income (loss), net of taxes
Accumulated other comprehensive loss, net of tax benefits	(719,000)			(20,039,000)
Total shareholders’ equity	2,323,231,000	$ 2,291,398,000	2,280,260,000	2,280,260,000	$ 2,242,689,000	$ 2,208,586,000	$ 2,183,515,000	$ 2,162,280,000
Total liabilities and shareholders’ equity	14,542,361,000		13,745,251,000	13,745,251,000
Other assets
Premises and equipment, net	5,232,177,000			5,109,628,000
Other	591,258,000			649,885,000
Other liabilities
Total other liabilities	586,853,000		$ 583,545,000	583,545,000
Balance Sheet related disclosures
Held-to-maturity investment securities	138,622,000			143,467,000
Securities sold under agreements to repurchase	96,000,000			115,000,000
Federal funds purchased	0			0
American Savings Bank (ASB)
Assets
Cash and due from banks	150,087,000			129,770,000
Interest-bearing deposits	10,918,000			48,628,000
Available-for-sale investment securities, at fair value	1,747,658,000			1,232,826,000
Held-to-maturity investment securities, at amortized cost	133,858,000			139,451,000
Stock in Federal Home Loan Bank, at cost	10,920,000			8,434,000
Loans held for investment	5,480,902,000			5,121,176,000
Allowance for credit losses	(91,459,000)			(53,355,000)
Net loans	5,389,443,000			5,067,821,000
Loans held for sale, at lower of cost or fair value	16,806,000			12,286,000
Other	533,865,000			511,611,000
Goodwill	82,190,000			82,190,000
Total assets	8,075,745,000			7,233,017,000
Liabilities
Deposit liabilities—noninterest-bearing	2,424,539,000			1,909,682,000
Deposit liabilities—interest-bearing	4,613,598,000			4,362,220,000
Other borrowings	151,875,000			115,110,000
Other	165,300,000			146,954,000
Total liabilities	7,355,312,000			6,533,966,000
Commitments and contingencies
Common stock	1,000			1,000
Additional paid-in capital	351,322,000			349,453,000
Retained earnings	356,812,000			358,259,000
Accumulated other comprehensive income (loss), net of taxes
Net unrealized gains on securities	22,248,000			2,481,000
Retirement benefit plans	(9,950,000)			(11,143,000)
Accumulated other comprehensive loss, net of tax benefits	12,298,000			(8,662,000)
Total shareholders’ equity	720,433,000			699,051,000
Total liabilities and shareholders’ equity	8,075,745,000			7,233,017,000
Other assets
Bank-owned life insurance	161,206,000			157,465,000
Premises and equipment, net	206,190,000			204,449,000
Accrued interest receivable	24,770,000			19,365,000
Mortgage-servicing rights	9,553,000			9,101,000
Low-income housing investments	71,467,000			66,302,000
Real estate acquired in settlement of loans, net	42,000			0
Other	60,637,000			54,929,000
Other	533,865,000			511,611,000
Other liabilities
Accrued expenses	52,170,000			45,822,000
Federal and state income taxes payable	9,750,000			14,996,000
Cashier’s checks	28,638,000			23,647,000
Advance payments by borrowers	5,413,000			10,486,000
Other	69,329,000			52,003,000
Total other liabilities	165,300,000			146,954,000
Balance Sheet related disclosures
Held-to-maturity investment securities	138,622,000			143,467,000
Securities sold under agreements to repurchase	95,900,000			115,000,000.0
Advances from the Federal Home Loan Bank	$ 56,000,000.0			$ 0