Bank segment - Balance sheet data (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Assets
Available-for-sale investment securities, at fair value	$ 1,387,571,000	$ 1,401,198,000
Held-to-maturity investment securities, at amortized cost	102,498,000	44,515,000
Stock in Federal Home Loan Bank, at cost	8,158,000	9,706,000
Allowance for loan losses	(54,127,000)	(53,637,000)
Net loans	4,700,232,000	4,617,131,000
Loans held for sale, at lower of cost or fair value	1,036,000	11,250,000
Other	596,481,000	513,535,000
Goodwill	82,190,000	82,190,000
Total assets	12,911,554,000	12,534,160,000
Liabilities
Other	545,862,000	521,018,000
Total liabilities	10,745,216,000	10,402,481,000
Commitments and contingencies
Retained earnings	527,802,000	476,836,000
Accumulated other comprehensive loss, net of tax benefits
Accumulated other comprehensive loss, net of tax benefits	(63,128,000)	(41,941,000)
Total shareholders’ equity	2,132,045,000	2,097,386,000	$ 2,103,022,000	$ 2,066,753,000
Total liabilities and shareholders’ equity	12,911,554,000	12,534,160,000
Other assets
Premises and equipment, net	4,694,101,000	4,460,248,000
Total Other Assets	596,481,000	513,535,000
Other liabilities
Total other liabilities	545,862,000	521,018,000
Balance Sheet related disclosures
Held-to-maturity investment securities	99,929,000	44,412,000
Securities sold under agreements to repurchase	71,000,000	141,000,000
American Savings Bank (ASB)
Assets
Cash and due from banks	119,453,000	140,934,000
Interest-bearing deposits	39,575,000	93,165,000
Available-for-sale investment securities, at fair value	1,387,571,000	1,401,198,000
Held-to-maturity investment securities, at amortized cost	102,498,000	44,515,000
Stock in Federal Home Loan Bank, at cost	8,158,000	9,706,000
Loans held for investment	4,754,359,000	4,670,768,000
Allowance for loan losses	(54,127,000)	(53,637,000)
Net loans	4,700,232,000	4,617,131,000
Loans held for sale, at lower of cost or fair value	1,036,000	11,250,000
Other	488,743,000	398,570,000
Goodwill	82,190,000	82,190,000
Total assets	6,929,456,000	6,798,659,000
Liabilities
Deposit liabilities—noninterest-bearing	1,789,351,000	1,760,233,000
Deposit liabilities—interest-bearing	4,341,064,000	4,130,364,000
Other borrowings	71,110,000	190,859,000
Other	115,401,000	110,356,000
Total liabilities	6,316,926,000	6,191,812,000
Commitments and contingencies
Common stock	1,000	1,000
Additional paid in capital	346,757,000	345,018,000
Retained earnings	317,519,000	292,957,000
Accumulated other comprehensive loss, net of tax benefits
Net unrealized losses on securities	(37,719,000)	(14,951,000)
Retirement benefit plans	(14,028,000)	(16,178,000)
Accumulated other comprehensive loss, net of tax benefits	(51,747,000)	(31,129,000)
Total shareholders’ equity	612,530,000	606,847,000
Total liabilities and shareholders’ equity	6,929,456,000	6,798,659,000
Other assets
Bank-owned life insurance	150,772,000	148,775,000
Premises and equipment, net	203,062,000	136,270,000
Prepaid expenses	5,477,000	3,961,000
Accrued interest receivable	19,818,000	18,724,000
Mortgage-servicing rights	8,426,000	8,639,000
Low-income housing equity investments	69,865,000	59,016,000
Real estate acquired in settlement of loans, net	438,000	133,000
Other	30,885,000	23,052,000
Total Other Assets	488,743,000	398,570,000
Other liabilities
Accrued expenses	56,830,000	39,312,000
Federal and state income taxes payable	1,287,000	3,736,000
Cashier’s checks	23,711,000	27,000,000
Advance payments by borrowers	4,998,000	10,245,000
Other	28,575,000	30,063,000
Total other liabilities	115,401,000	110,356,000
Balance Sheet related disclosures
Held-to-maturity investment securities	99,929,000	44,412,000
Securities sold under agreements to repurchase	71,000,000	141,000,000
Advances from Federal Home Loan Bank	$ 0	$ 50,000,000