Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Available-for-sale investment securities, at fair value	$ 1,210,748			$ 1,388,533
Held-to-maturity investment securities, at amortized cost	132,704			141,875
Stock in Federal Home Loan Bank, at cost	9,953			9,958
Allowance for loan losses	(53,040)			(52,119)
Net loans	5,031,296			4,790,902
Loans held for sale, at lower of cost or fair value	17,115			1,805
Other	576,263			530,364
Goodwill	82,190			82,190
Total assets	13,517,970			13,104,051
Liabilities
Other	539,987			580,788
Total liabilities	11,240,988			10,907,478
Commitments and contingencies
Retained earnings	590,651			543,623
Accumulated other comprehensive loss, net of tax benefits
Accumulated other comprehensive loss, net of tax benefits	(24,373)			(50,610)
Total shareholders’ equity	2,242,689	$ 2,208,586	$ 2,183,515	2,162,280	$ 2,132,045	$ 2,103,018	$ 2,091,777	$ 2,097,386
Total liabilities and shareholders’ equity	13,517,970			13,104,051
Other assets
Premises and equipment, net	5,006,394			4,830,118
Other	576,263			530,364
Other liabilities
Total other liabilities	539,987			580,788
Balance Sheet related disclosures
Held-to-maturity investment securities	137,497			142,057
Securities sold under agreements to repurchase	91,000			65,000
American Savings Bank (ASB)
Assets
Cash and due from banks	135,813			122,059
Interest-bearing deposits	1,315			4,225
Available-for-sale investment securities, at fair value	1,210,748			1,388,533
Held-to-maturity investment securities, at amortized cost	132,704			141,875
Stock in Federal Home Loan Bank, at cost	9,953			9,958
Loans held for investment	5,084,336			4,843,021
Allowance for loan losses	(53,040)			(52,119)
Net loans	5,031,296			4,790,902
Loans held for sale, at lower of cost or fair value	17,115			1,805
Other	514,116			486,347
Goodwill	82,190			82,190
Total assets	7,135,250			7,027,894
Liabilities
Deposit liabilities—noninterest-bearing	1,885,028			1,800,727
Deposit liabilities—interest-bearing	4,311,195			4,358,125
Other borrowings	129,190			110,040
Other	135,606			124,613
Total liabilities	6,461,019			6,393,505
Commitments and contingencies
Common stock	1			1
Additional paid-in capital	348,933			347,170
Retained earnings	339,029			325,286
Accumulated other comprehensive loss, net of tax benefits
Net unrealized gains (losses) on securities	2,945			(24,423)
Retirement benefit plans	(16,677)			(13,645)
Accumulated other comprehensive loss, net of tax benefits	(13,732)			(38,068)
Total shareholders’ equity	674,231			634,389
Total liabilities and shareholders’ equity	7,135,250			7,027,894
Other assets
Bank-owned life insurance	156,077			151,172
Premises and equipment, net	207,659			214,415
Accrued interest receivable	19,743			20,140
Mortgage-servicing rights	8,567			8,062
Low-income housing equity investments	69,286			67,626
Real estate acquired in settlement of loans, net	0			406
Real estate held for sale	9,074			0
Other	43,710			24,526
Other	514,116			486,347
Other liabilities
Accrued expenses	41,264			54,084
Federal and state income taxes payable	9,472			2,012
Cashier’s checks	27,498			26,906
Advance payments by borrowers	5,164			10,183
Other	52,208			31,428
Total other liabilities	135,606			124,613
Balance Sheet related disclosures
Held-to-maturity investment securities	137,497			0
Securities sold under agreements to repurchase	91,200			65,000
Advances from Federal Home Loan Bank	$ 38,000			$ 45,000