Bank segment - Components of Investment Securities (Details) $ in Thousands	Sep. 30, 2023 USD ($) issue	Dec. 31, 2022 USD ($) issue
Available-for-sale
Amortized cost	$ 1,559,205
Available-for-sale investment securities, at fair value	1,266,412	$ 1,429,667
Held-to-maturity
Held-to-maturity investment securities, at amortized cost	1,212,005	1,251,747
Held-to-maturity investment securities	1,052,221
American Savings Bank (ASB)
Available-for-sale
Amortized cost	1,559,205	1,678,205
Gross unrealized gains	0	0
Gross unrealized losses	(292,793)	(248,538)
Available-for-sale investment securities, at fair value	$ 1,266,412	$ 1,429,667
Gross unrealized losses, less than 12 months, number of issues | issue	5	129
Gross unrealized losses, less than 12 months, fair value	$ 12,650	$ 526,681
Gross unrealized losses, less than 12 months, amount	$ (3,432)	$ (61,147)
Gross unrealized losses, 12 months or longer, number of issues | issue	195	75
Gross unrealized losses, 12 months or longer, fair value	$ 1,239,268	$ 888,084
Gross unrealized losses, 12 months or more, amount	(289,361)	(187,391)
Held-to-maturity
Held-to-maturity investment securities, at amortized cost	1,212,005	1,251,747
Gross unrealized gains	0	2,670
Gross unrealized losses	(159,784)	(103,446)
Held-to-maturity investment securities	$ 1,052,221	$ 1,150,971
Less than 12 months, number of issues | issue	62	23
Less than 12 months, fair value	$ 612,587	$ 200,503
Less than 12 months, amount	$ (42,892)	$ (13,815)
12 months or longer, number of issues | issue	44	53
12 months or longer: fair value	$ 439,634	$ 601,792
12 months or longer, amount	(116,892)	(89,631)
U.S. Treasury and federal agency obligations | American Savings Bank (ASB)
Available-for-sale
Amortized cost	82,252	88,344
Gross unrealized gains	0	0
Gross unrealized losses	(7,252)	(7,281)
Available-for-sale investment securities, at fair value	$ 75,000	$ 81,063
Gross unrealized losses, less than 12 months, number of issues | issue	0	12
Gross unrealized losses, less than 12 months, fair value	$ 0	$ 41,201
Gross unrealized losses, less than 12 months, amount	$ 0	$ (2,120)
Gross unrealized losses, 12 months or longer, number of issues | issue	14	4
Gross unrealized losses, 12 months or longer, fair value	$ 75,000	$ 39,862
Gross unrealized losses, 12 months or more, amount	(7,252)	(5,161)
Held-to-maturity
Held-to-maturity investment securities, at amortized cost	59,912	59,894
Gross unrealized gains	0	0
Gross unrealized losses	(9,541)	(8,478)
Held-to-maturity investment securities	$ 50,371	$ 51,416
Less than 12 months, number of issues | issue	0	1
Less than 12 months, fair value	$ 0	$ 16,874
Less than 12 months, amount	$ 0	$ (3,222)
12 months or longer, number of issues | issue	3	2
12 months or longer: fair value	$ 50,371	$ 34,542
12 months or longer, amount	(9,541)	(5,256)
Mortgage-backed securities | American Savings Bank (ASB)
Available-for-sale
Amortized cost	1,427,186	1,530,582
Gross unrealized gains	0	0
Gross unrealized losses	(282,019)	(237,614)
Available-for-sale investment securities, at fair value	$ 1,145,167	$ 1,292,968
Gross unrealized losses, less than 12 months, number of issues | issue	5	113
Gross unrealized losses, less than 12 months, fair value	$ 12,650	$ 455,836
Gross unrealized losses, less than 12 months, amount	$ (3,432)	$ (56,999)
Gross unrealized losses, 12 months or longer, number of issues | issue	178	70
Gross unrealized losses, 12 months or longer, fair value	$ 1,132,517	$ 837,132
Gross unrealized losses, 12 months or more, amount	(278,587)	(180,615)
Held-to-maturity
Held-to-maturity investment securities, at amortized cost	1,152,093	1,191,853
Gross unrealized gains	0	2,670
Gross unrealized losses	(150,243)	(94,968)
Held-to-maturity investment securities	$ 1,001,850	$ 1,099,555
Less than 12 months, number of issues | issue	62	22
Less than 12 months, fair value	$ 612,587	$ 183,629
Less than 12 months, amount	$ (42,892)	$ (10,593)
12 months or longer, number of issues | issue	41	51
12 months or longer: fair value	$ 389,263	$ 567,250
12 months or longer, amount	(107,351)	(84,375)
Corporate bonds | American Savings Bank (ASB)
Available-for-sale
Amortized cost	35,273	44,377
Gross unrealized gains	0	0
Gross unrealized losses	(3,522)	(3,643)
Available-for-sale investment securities, at fair value	$ 31,751	$ 40,734
Gross unrealized losses, less than 12 months, number of issues | issue	0	4
Gross unrealized losses, less than 12 months, fair value	$ 0	$ 29,644
Gross unrealized losses, less than 12 months, amount	$ 0	$ (2,028)
Gross unrealized losses, 12 months or longer, number of issues | issue	3	1
Gross unrealized losses, 12 months or longer, fair value	$ 31,751	$ 11,090
Gross unrealized losses, 12 months or more, amount	(3,522)	(1,615)
Mortgage revenue bonds | American Savings Bank (ASB)
Available-for-sale
Amortized cost	14,494	14,902
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Available-for-sale investment securities, at fair value	$ 14,494	$ 14,902
Gross unrealized losses, less than 12 months, number of issues | issue	0	0
Gross unrealized losses, less than 12 months, fair value	$ 0	$ 0
Gross unrealized losses, less than 12 months, amount	$ 0	$ 0
Gross unrealized losses, 12 months or longer, number of issues | issue	0	0
Gross unrealized losses, 12 months or longer, fair value	$ 0	$ 0
Gross unrealized losses, 12 months or more, amount	$ 0	$ 0