Bank segment - Balance sheet data (Details) - USD ($)	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Assets
Available-for-sale investment securities, at fair value	$ 1,389,633,000		$ 1,232,826,000
Held-to-maturity investment securities, at amortized cost	124,623,000		139,451,000
Stock in Federal Home Loan Bank, at cost	9,880,000		8,434,000
Net loans	5,356,510,000	$ 5,067,821,000	5,067,821,000
Loans held for sale, at lower of cost or fair value	37,143,000		12,286,000
Other	556,793,000		649,885,000
Goodwill	82,190,000		82,190,000
Total assets	14,480,700,000	13,745,251,000	13,745,251,000
Liabilities
Other	580,082,000	583,545,000	583,545,000
Total liabilities	12,155,009,000	11,430,698,000	11,430,698,000
Commitments and contingencies
Retained earnings	616,941,000	622,042,000	622,042,000
Accumulated other comprehensive income (loss), net of taxes
Accumulated other comprehensive loss, net of tax benefits	(2,159,000)		(20,039,000)
Total shareholders’ equity	2,291,398,000	2,280,260,000	2,280,260,000	$ 2,208,586,000	$ 2,183,515,000	$ 2,162,280,000
Total liabilities and shareholders’ equity	14,480,700,000	13,745,251,000	13,745,251,000
Other assets
Premises and equipment, net	5,181,427,000		5,109,628,000
Other	556,793,000		649,885,000
Other liabilities
Total other liabilities	580,082,000	$ 583,545,000	583,545,000
Balance Sheet related disclosures
Held-to-maturity investment securities	131,131,000		143,467,000
Securities sold under agreements to repurchase	95,000,000		115,000,000
Federal funds purchased	0		0
American Savings Bank (ASB)
Assets
Cash and due from banks	140,968,000		129,770,000
Interest-bearing deposits	365,996,000		48,628,000
Available-for-sale investment securities, at fair value	1,389,633,000		1,232,826,000
Held-to-maturity investment securities, at amortized cost	124,623,000		139,451,000
Stock in Federal Home Loan Bank, at cost	9,880,000		8,434,000
Loans held for investment	5,437,817,000		5,121,176,000
Allowance for credit losses	(81,307,000)		(53,355,000)
Net loans	5,356,510,000		5,067,821,000
Loans held for sale, at lower of cost or fair value	37,143,000		12,286,000
Other	512,722,000		511,611,000
Goodwill	82,190,000		82,190,000
Total assets	8,019,665,000		7,233,017,000
Liabilities
Deposit liabilities—noninterest-bearing	2,422,042,000		1,909,682,000
Deposit liabilities—interest-bearing	4,607,910,000		4,362,220,000
Other borrowings	124,975,000		115,110,000
Other	158,344,000		146,954,000
Total liabilities	7,313,271,000		6,533,966,000
Commitments and contingencies
Common stock	1,000		1,000
Additional paid-in capital	350,826,000		349,453,000
Retained earnings	344,662,000		358,259,000
Accumulated other comprehensive income (loss), net of taxes
Net unrealized gains on securities	21,264,000		2,481,000
Retirement benefit plans	(10,359,000)		(11,143,000)
Accumulated other comprehensive loss, net of tax benefits	10,905,000		(8,662,000)
Total shareholders’ equity	706,394,000		699,051,000
Total liabilities and shareholders’ equity	8,019,665,000		7,233,017,000
Other assets
Bank-owned life insurance	159,951,000		157,465,000
Premises and equipment, net	203,217,000		204,449,000
Accrued interest receivable	23,381,000		19,365,000
Mortgage-servicing rights	9,647,000		9,101,000
Low-income housing investments	61,632,000		66,302,000
Real estate acquired in settlement of loans, net	43,000		0
Other	54,851,000		54,929,000
Other	512,722,000		511,611,000
Other liabilities
Accrued expenses	40,382,000		45,822,000
Federal and state income taxes payable	18,021,000		14,996,000
Cashier’s checks	25,284,000		23,647,000
Advance payments by borrowers	10,458,000		10,486,000
Other	64,199,000		52,003,000
Total other liabilities	158,344,000		146,954,000
Balance Sheet related disclosures
Held-to-maturity investment securities	131,131,000		143,467,000
Securities sold under agreements to repurchase	95,000,000.0		115,000,000
Advances from the Federal Home Loan Bank	$ 30,000,000.0		$ 0