Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Assets
Available-for-sale investment securities, at fair value	$ 693,520	$ 550,394
Stock in Federal Home Loan Bank, at cost	10,678	69,302
Loans receivable held for investment, net	4,410,817	4,389,033
Loans held for sale, at lower of cost or fair value	5,581	8,424
Other	493,151	542,523
Goodwill	82,190	82,190
Total assets	11,439,631	11,185,142
Liabilities
Other	461,335	531,230
Total liabilities	$ 9,506,818	$ 9,360,276
Commitments and contingencies
Equity [Abstract]
Retained earnings	$ 298,398	$ 296,654		$ 255,030
Accumulated other comprehensive income (loss), net of taxes:
Accumulated other comprehensive loss, net of tax benefits	(26,447)	(27,378)	$ (12,683)	(16,750)
Total shareholders’ equity	1,898,520	1,790,573	$ 1,759,935	$ 1,726,406
Total liabilities and shareholders’ equity	11,439,631	11,185,142
Other assets
Property, plant and equipment, net	4,269,241	4,148,774
Total Other Assets	493,151	542,523
Other liabilities
Total other liabilities	461,335	531,230
Balance Sheet related disclosures
Securities sold under agreements to repurchase	214,000	191,000
American Savings Bank (ASB)
Assets
Cash and due from banks	106,914	107,233
Interest-bearing deposits	162,088	54,230
Available-for-sale investment securities, at fair value	693,520	550,394
Stock in Federal Home Loan Bank, at cost	10,678	69,302
Loans receivable held for investment	4,457,182	4,434,651
Allowance for loan losses	(46,365)	(45,618)
Loans receivable held for investment, net	4,410,817	4,389,033
Loans held for sale, at lower of cost or fair value	5,581	8,424
Other	305,310	305,416
Goodwill	82,190	82,190
Total assets	5,777,098	5,566,222
Liabilities
Deposit liabilities-noninterest-bearing	1,455,721	1,342,794
Deposit liabilities-interest-bearing	3,347,550	3,280,621
Other borrowings	314,157	290,656
Other	113,015	118,363
Total liabilities	$ 5,230,443	$ 5,032,434
Commitments and contingencies
Equity [Abstract]
Common stock	$ 1	$ 1
Additional paid in capital	339,416	338,411
Retained earnings	223,260	211,934
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized losses on securities	219	462
Accumulated other comprehensive loss, net of tax benefits	(16,241)	(17,020)
Accumulated other comprehensive loss, net of tax benefits	(16,022)	(16,558)
Total shareholders’ equity	546,655	533,788
Total liabilities and shareholders’ equity	5,777,098	5,566,222
Other assets
Bank-owned life insurance	136,062	134,115
Property, plant and equipment, net	85,976	92,407
Prepaid expenses	3,728	3,196
Accrued interest receivable	14,052	13,632
Mortgage-servicing rights	12,265	11,540
Low-income housing equity investments	30,974	33,438
Real estate acquired in settlement of loans, net	318	891
Other	21,935	16,197
Total Other Assets	305,310	305,416
Other liabilities
Accrued expenses	26,915	37,880
Federal and state income taxes payable	26,502	28,642
Cashier's checks	27,670	20,509
Advance payments by borrowers	10,093	9,652
Other	21,835	21,680
Total other liabilities	113,015	118,363
Balance Sheet related disclosures
Securities sold under agreements to repurchase	214,000	191,000
Advances from Federal Home Loan Bank	$ 100,000	$ 100,000