Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Assets
Available-for-sale investment securities, at fair value	$ 785,837	$ 550,394
Stock in Federal Home Loan Bank, at cost	10,678	69,302
Net loans	4,487,130	4,389,033
Loans held for sale, at lower of cost or fair value	5,598	8,424
Other	453,099	542,523
Goodwill	82,190	82,190
Total assets	11,573,466	11,185,142
Liabilities
Other	456,726	531,230
Total liabilities	$ 9,618,172	$ 9,360,276
Commitments and contingencies
Equity [Abstract]
Retained earnings	$ 315,759	$ 296,654
Accumulated other comprehensive income (loss), net of taxes:
Accumulated other comprehensive loss, net of tax benefits	(22,017)	(27,378)	$ (13,994)	$ (16,750)
Total shareholders’ equity	1,921,001	1,790,573	$ 1,800,457	$ 1,726,406
Total liabilities and shareholders’ equity	11,573,466	11,185,142
Other assets
Premises and equipment, net	4,317,121	4,148,774
Total Other Assets	453,099	542,523
Other liabilities
Total other liabilities	456,726	531,230
Balance Sheet related disclosures
Securities sold under agreements to repurchase	269,000	191,000
American Savings Bank (ASB)
Assets
Cash and due from banks	103,934	107,233
Interest-bearing deposits	73,041	54,230
Available-for-sale investment securities, at fair value	785,837	550,394
Stock in Federal Home Loan Bank, at cost	10,678	69,302
Loans receivable held for investment	4,535,404	4,434,651
Allowance for loan losses	(48,274)	(45,618)
Net loans	4,487,130	4,389,033
Loans held for sale, at lower of cost or fair value	5,598	8,424
Other	307,089	305,416
Goodwill	82,190	82,190
Total assets	5,855,497	5,566,222
Liabilities
Deposit liabilities—noninterest-bearing	1,422,843	1,342,794
Deposit liabilities—interest-bearing	3,403,111	3,280,621
Other borrowings	368,593	290,656
Other	103,553	118,363
Total liabilities	$ 5,298,100	$ 5,032,434
Commitments and contingencies
Equity [Abstract]
Common stock	$ 1	$ 1
Additional paid in capital	339,980	338,411
Retained earnings	229,211	211,934
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized gains on securities	4,070	462
Retirement benefit plans	(15,865)	(17,020)
Accumulated other comprehensive loss, net of tax benefits	(11,795)	(16,558)
Total shareholders’ equity	557,397	533,788
Total liabilities and shareholders’ equity	5,855,497	5,566,222
Other assets
Bank-owned life insurance	136,969	134,115
Premises and equipment, net	87,432	92,407
Prepaid expenses	3,879	3,196
Accrued interest receivable	14,577	13,632
Mortgage-servicing rights	12,258	11,540
Low-income housing equity investments	34,323	33,438
Real estate acquired in settlement of loans, net	247	891
Other	17,404	16,197
Total Other Assets	307,089	305,416
Other liabilities
Accrued expenses	28,952	37,880
Federal and state income taxes payable	21,565	28,642
Cashier’s checks	25,852	20,509
Advance payments by borrowers	5,389	9,652
Other	21,795	21,680
Total other liabilities	103,553	118,363
Balance Sheet related disclosures
Securities sold under agreements to repurchase	269,000	191,000
Advances from Federal Home Loan Bank	$ 100,000	$ 100,000