Bank segment - Components of investment securities (Details) $ in Thousands	Mar. 31, 2021 USD ($) issue	Dec. 31, 2020 USD ($) issue
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 2,340,488
Estimated fair value	2,305,257
Debt Securities, Held-to-maturity [Abstract]
Amortized cost	295,046	$ 226,947
Held-to-maturity investment securities	285,599	229,963
American Savings Bank (ASB)
Debt Securities, Available-for-sale [Abstract]
Amortized cost	2,340,488	1,943,114
Gross unrealized gains	18,173	30,454
Gross unrealized losses	(53,404)	(3,151)
Estimated fair value	$ 2,305,257	$ 1,970,417
Gross unrealized losses, Less than 12 months, Number of issues \| issue	93	22
Gross unrealized losses, Less than 12 months, Fair value	$ 1,617,916	$ 373,924
Gross unrealized losses, Less than 12 months, Amount	$ (53,385)	$ (3,151)
Gross unrealized losses, 12 months or longer, Number of issues \| issue	1	0
Gross unrealized losses, 12 months or longer, Fair value	$ 958	$ 0
Gross unrealized losses, 12 months or more, amount	(19)	0
Debt Securities, Held-to-maturity [Abstract]
Amortized cost	295,046	226,947
Gross unrealized gains	2,812	3,846
Gross unrealized losses	(12,259)	(830)
Held-to-maturity investment securities	$ 285,599	$ 229,963
Less than 12 months: Number of issues \| issue	16	7
Less than 12 Months: Fair value	$ 220,908	$ 114,152
Less than 12 Months: Amount	$ (12,259)	$ (830)
12 months or longer: Number of issues \| issue	0	0
12 months or longer: Fair value	$ 0	$ 0
12 months or longer: Amount	0	0
U.S. Treasury and federal agency obligations \| American Savings Bank (ASB)
Debt Securities, Available-for-sale [Abstract]
Amortized cost	55,969	60,260
Gross unrealized gains	1,719	2,062
Gross unrealized losses	(41)	0
Estimated fair value	$ 57,647	$ 62,322
Gross unrealized losses, Less than 12 months, Number of issues \| issue	1	0
Gross unrealized losses, Less than 12 months, Fair value	$ 4,908	$ 0
Gross unrealized losses, Less than 12 months, Amount	$ (41)	$ 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
Mortgage-backed securities* \| American Savings Bank (ASB)
Debt Securities, Available-for-sale [Abstract]
Amortized cost	2,239,311	1,825,893
Gross unrealized gains	15,404	26,817
Gross unrealized losses	(53,363)	(3,151)
Estimated fair value	$ 2,201,352	$ 1,849,559
Gross unrealized losses, Less than 12 months, Number of issues \| issue	92	22
Gross unrealized losses, Less than 12 months, Fair value	$ 1,613,008	$ 373,924
Gross unrealized losses, Less than 12 months, Amount	$ (53,344)	$ (3,151)
Gross unrealized losses, 12 months or longer, Number of issues \| issue	1	0
Gross unrealized losses, 12 months or longer, Fair value	$ 958	$ 0
Gross unrealized losses, 12 months or more, amount	(19)	0
Debt Securities, Held-to-maturity [Abstract]
Amortized cost	295,046	226,947
Gross unrealized gains	2,812	3,846
Gross unrealized losses	(12,259)	(830)
Held-to-maturity investment securities	$ 285,599	$ 229,963
Less than 12 months: Number of issues \| issue	16	7
Less than 12 Months: Fair value	$ 220,908	$ 114,152
Less than 12 Months: Amount	$ (12,259)	$ (830)
12 months or longer: Number of issues \| issue	0	0
12 months or longer: Fair value	$ 0	$ 0
12 months or longer: Amount	0	0
Corporate bonds \| American Savings Bank (ASB)
Debt Securities, Available-for-sale [Abstract]
Amortized cost	29,781	29,776
Gross unrealized gains	1,050	1,575
Gross unrealized losses	0	0
Estimated fair value	$ 30,831	$ 31,351
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
Mortgage revenue bonds \| American Savings Bank (ASB)
Debt Securities, Available-for-sale [Abstract]
Amortized cost	15,427	27,185
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	$ 15,427	$ 27,185
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

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Held-to-maturity Securities, More Than 12 Months, Number of Issues

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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