Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Assets
Available-for-sale investment securities, at fair value	$ 1,228,922	$ 1,105,182
Stock in Federal Home Loan Bank, at cost	11,706	11,218
Net loans	4,669,274	4,683,160
Loans held for sale, at lower of cost or fair value	10,454	18,817
Other	467,160	447,621
Goodwill	82,190	82,190
Total assets	12,543,275	12,425,506
Liabilities
Other	423,989	473,512
Total liabilities	10,443,394	10,324,460
Commitments and contingencies
Equity [Abstract]
Retained earnings	439,452	438,972
Accumulated other comprehensive income (loss), net of taxes:
Accumulated other comprehensive loss, net of tax benefits	(32,144)	(33,129)
Total shareholders’ equity	2,065,588	2,066,753	$ 1,942,179	$ 1,927,640
Total liabilities and shareholders’ equity	12,543,275	12,425,506
Other assets
Premises and equipment, net	4,641,514	4,603,465
Total Other Assets	467,160	447,621
Other liabilities
Total other liabilities	423,989	473,512
Balance Sheet related disclosures
Securities sold under agreements to repurchase	100,000	93,000
American Savings Bank (ASB)
Assets
Cash and due from banks	125,901	137,083
Interest-bearing deposits	94,573	52,128
Restricted cash	0	1,764
Available-for-sale investment securities, at fair value	1,228,922	1,105,182
Stock in Federal Home Loan Bank, at cost	11,706	11,218
Loans receivable held for investment	4,725,271	4,738,693
Allowance for loan losses	(55,997)	(55,533)
Net loans	4,669,274	4,683,160
Loans held for sale, at lower of cost or fair value	10,454	18,817
Other	336,626	329,815
Goodwill	82,190	82,190
Total assets	6,559,646	6,421,357
Liabilities
Deposit liabilities—noninterest-bearing	1,696,390	1,639,051
Deposit liabilities—interest-bearing	3,978,700	3,909,878
Other borrowings	200,154	192,618
Other	98,223	101,635
Total liabilities	5,973,467	5,843,182
Commitments and contingencies
Common stock	1	1
Equity [Abstract]
Additional paid in capital	343,435	342,704
Retained earnings	264,381	257,943
Accumulated other comprehensive income (loss), net of taxes:
Net unrealized losses on securities	(7,708)	(7,931)
Retirement benefit plans	(13,930)	(14,542)
Accumulated other comprehensive loss, net of tax benefits	(21,638)	(22,473)
Total shareholders’ equity	586,179	578,175
Total liabilities and shareholders’ equity	6,559,646	6,421,357
Other assets
Bank-owned life insurance	144,661	143,197
Premises and equipment, net	94,865	90,570
Prepaid expenses	4,031	3,348
Accrued interest receivable	16,508	16,824
Mortgage-servicing rights	9,294	9,373
Low-income housing equity investments	46,782	47,081
Real estate acquired in settlement of loans, net	1,242	1,189
Other	19,243	18,233
Total Other Assets	336,626	329,815
Other liabilities
Accrued expenses	32,324	36,754
Federal and state income taxes payable	10,642	4,728
Cashier’s checks	23,777	24,156
Advance payments by borrowers	6,134	10,335
Other	25,346	25,662
Total other liabilities	98,223	101,635
Balance Sheet related disclosures
Securities sold under agreements to repurchase	100,000	93,000
Advances from Federal Home Loan Bank	$ 100,000	$ 100,000