Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Assets
Available-for-sale investment securities, at fair value	$ 1,418,490	$ 1,401,198
Held-to-maturity investment securities, at amortized cost	43,450	44,515
Stock in Federal Home Loan Bank, at cost	10,158	9,706
Allowance for loan losses	(53,895)	(53,637)
Net loans	4,688,129	4,617,131
Loans held for sale, at lower of cost or fair value	7,379	11,250
Other	526,744	513,535
Goodwill	82,190	82,190
Total assets	12,702,718	12,534,160
Liabilities
Other	475,822	521,018
Total liabilities	10,576,648	10,402,481
Commitments and contingencies
Retained earnings	483,342	476,836
Accumulated other comprehensive loss, net of tax benefits
Accumulated other comprehensive loss, net of tax benefits	(54,714)	(41,941)
Total shareholders’ equity	2,091,777	2,097,386	$ 2,065,588	$ 2,066,753
Total liabilities and shareholders’ equity	12,702,718	12,534,160
Other assets
Premises and equipment, net	4,542,558	4,460,248
Total Other Assets	526,744	513,535
Other liabilities
Total other liabilities	475,822	521,018
Balance Sheet related disclosures
Held-to-maturity investment securities	42,491	44,412
Securities sold under agreements to repurchase	50,000	141,000
American Savings Bank (ASB)
Assets
Cash and due from banks	123,580	140,934
Interest-bearing deposits	90,643	93,165
Available-for-sale investment securities, at fair value	1,418,490	1,401,198
Held-to-maturity investment securities, at amortized cost	43,450	44,515
Stock in Federal Home Loan Bank, at cost	10,158	9,706
Loans held for investment	4,742,024	4,670,768
Allowance for loan losses	(53,895)	(53,637)
Net loans	4,688,129	4,617,131
Loans held for sale, at lower of cost or fair value	7,379	11,250
Other	425,426	398,570
Goodwill	82,190	82,190
Total assets	6,889,445	6,798,659
Liabilities
Deposit liabilities—noninterest-bearing	1,795,114	1,760,233
Deposit liabilities—interest-bearing	4,283,953	4,130,364
Other borrowings	100,430	190,859
Other	106,482	110,356
Total liabilities	6,285,979	6,191,812
Commitments and contingencies
Common stock	1	1
Additional paid in capital	345,652	345,018
Retained earnings	300,837	292,957
Accumulated other comprehensive loss, net of tax benefits
Net unrealized losses on securities	(28,248)	(14,951)
Retirement benefit plans	(14,776)	(16,178)
Accumulated other comprehensive loss, net of tax benefits	(43,024)	(31,129)
Total shareholders’ equity	603,466	606,847
Total liabilities and shareholders’ equity	6,889,445	6,798,659
Other assets
Bank-owned life insurance	149,656	148,775
Premises and equipment, net	164,702	136,270
Prepaid expenses	4,549	3,961
Accrued interest receivable	18,461	18,724
Mortgage-servicing rights	8,541	8,639
Low-income housing equity investments	57,222	59,016
Real estate acquired in settlement of loans, net	0	133
Other	22,295	23,052
Total Other Assets	425,426	398,570
Other liabilities
Accrued expenses	49,034	39,312
Federal and state income taxes payable	1,369	3,736
Cashier’s checks	22,990	27,000
Advance payments by borrowers	6,255	10,245
Other	26,834	30,063
Total other liabilities	106,482	110,356
Balance Sheet related disclosures
Held-to-maturity investment securities	42,491	44,412
Securities sold under agreements to repurchase	50,000	141,000
Advances from Federal Home Loan Bank	$ 50,000	$ 50,000