Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Assets
Cash and cash equivalents	$ 277,579	$ 341,421	$ 205,514
Investment securities
Available-for-sale investment securities, at fair value	2,305,257	1,970,417
Held-to-maturity investment securities, at amortized cost	295,046	226,947
Stock in Federal Home Loan Bank, at cost	10,000	8,680
Collateral-dependent loans, amortized cost	5,325,770	5,345,226
Allowance for credit losses	(91,793)	(101,201)	(77,084)	$ (53,355)
Total loans, net	5,218,288	5,232,642
Loans held for sale, at lower of cost or fair value	23,637	28,275
Other	639,505	629,149
Goodwill	82,190	82,190
Total assets	15,358,673	15,004,007
Liabilities and shareholder’s equity
Other	557,758	618,438
Total liabilities	13,003,087	12,632,212
Commitments and contingencies
Retained earnings	687,600	660,398
Accumulated other comprehensive income (loss), net of taxes
Accumulated other comprehensive loss, net of tax benefits	(45,280)	(1,264)
Total shareholders’ equity	2,321,293	2,337,502	$ 2,276,505	$ 2,280,260
Total liabilities and shareholders’ equity	15,358,673	15,004,007
Other assets
Premises and equipment, net	5,283,768	5,265,735
Other	639,505	629,149
Other liabilities
Total other liabilities	557,758	618,438
Balance Sheet related disclosures
Securities sold under agreements to repurchase	103,000	90,000
Federal funds purchased	0	0
Held-to-maturity investment securities	285,599	229,963
American Savings Bank (ASB)
Assets
Cash and due from banks	113,698	178,422
Interest-bearing deposits	110,365	114,304
Cash and cash equivalents	224,063	292,726
Investment securities
Available-for-sale investment securities, at fair value	2,305,257	1,970,417
Held-to-maturity investment securities, at amortized cost	295,046	226,947
Stock in Federal Home Loan Bank, at cost	10,000	8,680
Collateral-dependent loans, amortized cost	5,310,081	5,333,843
Allowance for credit losses	(91,793)	(101,201)
Total loans, net	5,218,288	5,232,642
Loans held for sale, at lower of cost or fair value	23,637	28,275
Other	559,543	554,656
Goodwill	82,190	82,190
Total assets	8,718,024	8,396,533
Liabilities and shareholder’s equity
Deposit liabilities—noninterest-bearing	2,833,844	2,598,500
Deposit liabilities—interest-bearing	4,911,450	4,788,457
Other borrowings	102,685	89,670
Other	154,418	183,731
Total liabilities	8,002,397	7,660,358
Commitments and contingencies
Common stock	1	1
Additional paid-in capital	352,408	351,758
Retained earnings	394,026	369,470
Accumulated other comprehensive income (loss), net of taxes
Net unrealized gains (losses) on securities	(25,791)	19,986
Retirement benefit plans	(5,017)	(5,040)
Accumulated other comprehensive loss, net of tax benefits	(30,808)	14,946
Total shareholders’ equity	715,627	736,175
Total liabilities and shareholders’ equity	8,718,024	8,396,533
Other assets
Bank-owned life insurance	162,821	163,265
Premises and equipment, net	206,247	206,134
Accrued interest receivable	24,381	24,616
Mortgage-servicing rights	10,685	10,020
Low-income housing investments	80,791	83,435
Other	74,618	67,186
Other	559,543	554,656
Other liabilities
Accrued expenses	55,073	62,694
Federal and state income taxes payable	1,709	6,582
Cashier’s checks	29,363	38,011
Advance payments by borrowers	5,863	10,207
Other	62,410	66,237
Total other liabilities	154,418	183,731
Balance Sheet related disclosures
Securities sold under agreements to repurchase	102,700	89,700
Advances from the Federal Home Loan Bank	0	0
Held-to-maturity investment securities	$ 285,599	$ 229,963