Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Assets
Available-for-sale investment securities, at fair value	$ 1,302,886	$ 1,105,182
Stock in Federal Home Loan Bank, at cost	11,706	11,218
Allowance for loan losses	(56,356)	(55,533)
Net loans	4,688,278	4,683,160
Loans held for sale, at lower of cost or fair value	5,261	18,817
Other	478,763	447,621
Goodwill	82,190	82,190
Total assets	12,693,805	12,425,506
Liabilities
Other	434,610	473,512
Total liabilities	10,584,483	10,324,460
Commitments and contingencies
Retained earnings	444,400	438,972
Accumulated other comprehensive loss, net of tax benefits
Accumulated other comprehensive loss, net of tax benefits	(29,774)	(33,129)
Total shareholders’ equity	2,075,029	2,066,753	$ 1,966,182	$ 1,927,640
Total liabilities and shareholders’ equity	12,693,805	12,425,506
Other assets
Premises and equipment, net	4,726,524	4,603,465
Total Other Assets	478,763	447,621
Other liabilities
Total other liabilities	434,610	473,512
Balance Sheet related disclosures
Securities sold under agreements to repurchase	88,000	93,000
American Savings Bank (ASB)
Assets
Cash and due from banks	128,609	137,083
Interest-bearing deposits	37,049	52,128
Restricted cash	0	1,764
Available-for-sale investment securities, at fair value	1,302,886	1,105,182
Stock in Federal Home Loan Bank, at cost	11,706	11,218
Loans receivable held for investment	4,744,634	4,738,693
Allowance for loan losses	(56,356)	(55,533)
Net loans	4,688,278	4,683,160
Loans held for sale, at lower of cost or fair value	5,261	18,817
Other	354,898	329,815
Goodwill	82,190	82,190
Total assets	6,610,877	6,421,357
Liabilities
Deposit liabilities—noninterest-bearing	1,694,150	1,639,051
Deposit liabilities—interest-bearing	4,030,236	3,909,878
Other borrowings	188,130	192,618
Other	101,974	101,635
Total liabilities	6,014,490	5,843,182
Commitments and contingencies
Common stock	1	1
Additional paid in capital	344,062	342,704
Retained earnings	271,739	257,943
Accumulated other comprehensive loss, net of tax benefits
Net unrealized losses on securities	(5,687)	(7,931)
Retirement benefit plans	(13,728)	(14,542)
Accumulated other comprehensive loss, net of tax benefits	(19,415)	(22,473)
Total shareholders’ equity	596,387	578,175
Total liabilities and shareholders’ equity	6,610,877	6,421,357
Other assets
Bank-owned life insurance	146,122	143,197
Premises and equipment, net	108,158	90,570
Prepaid expenses	4,632	3,348
Accrued interest receivable	16,949	16,824
Mortgage-servicing rights	9,181	9,373
Low-income housing equity investments	48,596	47,081
Real estate acquired in settlement of loans, net	1,554	1,189
Other	19,706	18,233
Total Other Assets	354,898	329,815
Other liabilities
Accrued expenses	34,451	36,754
Federal and state income taxes payable	6,336	4,728
Cashier’s checks	24,191	24,156
Advance payments by borrowers	10,334	10,335
Other	26,662	25,662
Total other liabilities	101,974	101,635
Balance Sheet related disclosures
Securities sold under agreements to repurchase	88,000	93,000
Advances from Federal Home Loan Bank	$ 100,000	$ 100,000