Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Assets
Available-for-sale investment securities, at fair value	$ 1,320,110	$ 1,105,182
Stock in Federal Home Loan Bank, at cost	9,706	11,218
Allowance for loan losses	(53,047)	(55,533)
Net loans	4,623,234	4,683,160
Loans held for sale, at lower of cost or fair value	15,728	18,817
Other	474,444	447,621
Goodwill	82,190	82,190
Total assets	12,742,850	12,425,506
Liabilities
Other	460,396	473,512
Total liabilities	10,605,535	10,324,460
Commitments and contingencies
Retained earnings	470,750	438,972
Accumulated other comprehensive loss, net of tax benefits
Accumulated other comprehensive loss, net of tax benefits	(29,220)	(33,129)
Total shareholders’ equity	2,103,022	2,066,753	$ 2,068,388	$ 1,927,640
Total liabilities and shareholders’ equity	12,742,850	12,425,506
Other assets
Premises and equipment, net	4,813,875	4,603,465
Total Other Assets	474,444	447,621
Other liabilities
Total other liabilities	460,396	473,512
Balance Sheet related disclosures
Securities sold under agreements to repurchase	104,000	93,000
American Savings Bank (ASB)
Assets
Cash and due from banks	120,492	137,083
Interest-bearing deposits	69,223	52,128
Restricted cash	0	1,764
Available-for-sale investment securities, at fair value	1,320,110	1,105,182
Stock in Federal Home Loan Bank, at cost	9,706	11,218
Loans receivable held for investment	4,676,281	4,738,693
Allowance for loan losses	(53,047)	(55,533)
Net loans	4,623,234	4,683,160
Loans held for sale, at lower of cost or fair value	15,728	18,817
Other	378,224	329,815
Goodwill	82,190	82,190
Total assets	6,618,907	6,421,357
Liabilities
Deposit liabilities—noninterest-bearing	1,710,698	1,639,051
Deposit liabilities—interest-bearing	4,041,628	3,909,878
Other borrowings	153,552	192,618
Other	107,558	101,635
Total liabilities	6,013,436	5,843,182
Commitments and contingencies
Common stock	1	1
Additional paid in capital	344,512	342,704
Retained earnings	279,956	257,943
Accumulated other comprehensive loss, net of tax benefits
Net unrealized losses on securities	(5,479)	(7,931)
Retirement benefit plans	(13,519)	(14,542)
Accumulated other comprehensive loss, net of tax benefits	(18,998)	(22,473)
Total shareholders’ equity	605,471	578,175
Total liabilities and shareholders’ equity	6,618,907	6,421,357
Other assets
Bank-owned life insurance	147,391	143,197
Premises and equipment, net	123,326	90,570
Prepaid expenses	5,356	3,348
Accrued interest receivable	17,488	16,824
Mortgage-servicing rights	9,070	9,373
Low-income housing equity investments	54,515	47,081
Real estate acquired in settlement of loans, net	1,183	1,189
Other	19,895	18,233
Total Other Assets	378,224	329,815
Other liabilities
Accrued expenses	41,698	36,754
Federal and state income taxes payable	6,829	4,728
Cashier’s checks	27,448	24,156
Advance payments by borrowers	4,867	10,335
Other	26,716	25,662
Total other liabilities	107,558	101,635
Balance Sheet related disclosures
Securities sold under agreements to repurchase	104,000	93,000
Advances from Federal Home Loan Bank	$ 50,000	$ 100,000