Bank segment - Balance sheet data (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Assets
Cash and cash equivalents	$ 247,443		$ 341,421	$ 574,482
Investment securities
Available-for-sale investment securities, at fair value	2,509,906		1,970,417
Held-to-maturity investment securities, at amortized cost	375,655		226,947
Held-to-maturity investment securities	374,141		229,963
Stock in Federal Home Loan Bank, at cost	10,000		8,680
Allowance for credit losses	(78,252)	$ (91,793)	(101,201)	(81,307)	$ (77,084)	$ (53,355)
Total loans, net	5,106,207		5,232,642
Loans held for sale, at lower of cost or fair value	50,877		28,275
Other assets	668,368		629,149
Goodwill	82,190		82,190
Total assets	15,606,729		15,004,007
Liabilities and shareholder’s equity
Other	584,610		618,438
Total liabilities	13,205,107		12,632,212
Commitments and contingencies
Retained earnings	714,317		660,398
Accumulated other comprehensive income (loss), net of taxes
Accumulated other comprehensive loss, net of tax benefits	(28,808)		(1,264)
Total shareholders’ equity	2,367,329	$ 2,321,293	2,337,502	$ 2,291,398	$ 2,276,505	$ 2,280,260
Total liabilities and shareholders’ equity	15,606,729		15,004,007
Other assets
Premises and equipment, net	5,310,162		5,265,735
Other assets	668,368		629,149
Other liabilities
Other liabilities	584,610		618,438
American Savings Bank (ASB)
Assets
Cash and due from banks	115,567		178,422
Interest-bearing deposits	105,800		114,304
Cash and cash equivalents	221,367		292,726
Investment securities
Available-for-sale investment securities, at fair value	2,509,906		1,970,417
Held-to-maturity investment securities, at amortized cost	375,655		226,947
Held-to-maturity investment securities	374,141		229,963
Stock in Federal Home Loan Bank, at cost	10,000		8,680
Collateral-dependent loans, amortized cost	5,184,459		5,333,843
Allowance for credit losses	(78,252)		(101,201)
Total loans, net	5,106,207		5,232,642
Loans held for sale, at lower of cost or fair value	50,877		28,275
Other assets	553,702		554,656
Goodwill	82,190		82,190
Total assets	8,909,904		8,396,533
Liabilities and shareholder’s equity
Deposit liabilities—noninterest-bearing	2,868,770		2,598,500
Deposit liabilities—interest-bearing	5,004,660		4,788,457
Other borrowings	129,665		89,670
Other	166,419		183,731
Total liabilities	8,169,514		7,660,358
Commitments and contingencies
Common stock	1		1
Additional paid-in capital	352,888		351,758
Retained earnings	401,310		369,470
Accumulated other comprehensive income (loss), net of taxes
Net unrealized gains (losses) on securities	(8,815)		19,986
Retirement benefit plans	(4,994)		(5,040)
Accumulated other comprehensive loss, net of tax benefits	(13,809)		14,946
Total shareholders’ equity	740,390		736,175
Total liabilities and shareholders’ equity	8,909,904		8,396,533
Other assets
Bank-owned life insurance	164,453		163,265
Premises and equipment, net	205,917		206,134
Accrued interest receivable	23,064		24,616
Mortgage-servicing rights	10,754		10,020
Low-income housing investments	87,371		83,435
Other	62,143		67,186
Other assets	553,702		554,656
Other liabilities
Accrued expenses	61,156		62,694
Federal and state income taxes payable	1,508		6,582
Cashier’s checks	30,818		38,011
Advance payments by borrowers	10,374		10,207
Other	62,563		66,237
Other liabilities	$ 166,419		$ 183,731