Bank segment - Components of investment securities (Details) $ in Thousands	Sep. 30, 2020 USD ($) issues	Dec. 31, 2019 USD ($) issues
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 1,717,265	$ 1,229,438
Gross unrealized gains	31,374	8,015
Gross unrealized losses	(981)	(4,627)
Estimated fair value	$ 1,747,658	$ 1,232,826
Gross unrealized losses, Less than 12 months, Number of issues \| issues	17	21
Gross unrealized losses, Less than 12 months, Fair value	$ 304,930	$ 156,181
Gross unrealized losses, Less than 12 months, Amount	$ (981)	$ (830)
Gross unrealized losses, 12 months or longer, Number of issues \| issues	0	78
Gross unrealized losses, 12 months or longer, Fair value	$ 0	$ 345,657
Gross unrealized losses, 12 months or longer, Amount	0	(3,797)
Debt Securities, Held-to-maturity [Abstract]
Amortized cost	133,858	139,451
Gross unrealized gains	4,878	4,087
Gross unrealized losses	(114)	(71)
Held-to-maturity investment securities	$ 138,622	$ 143,467
Less Than 12 Months: Number Of Issues \| issues	2	1
Less than 12 Months: Fair value	$ 28,486	$ 12,986
Less than 12 Months: Amount	$ (114)	$ (71)
12 months or longer: Number of issues \| issues	0	0
12 months or longer: Fair value	$ 0	$ 0
12 months or longer: Amount	0	0
U.S. Treasury and federal agency obligations
Debt Securities, Available-for-sale [Abstract]
Amortized cost	61,359	117,255
Gross unrealized gains	2,229	652
Gross unrealized losses	0	(120)
Estimated fair value	$ 63,588	$ 117,787
Gross unrealized losses, Less than 12 months, Number of issues \| issues	0	2
Gross unrealized losses, Less than 12 months, Fair value	$ 0	$ 4,110
Gross unrealized losses, Less than 12 months, Amount	$ 0	$ (11)
Gross unrealized losses, 12 months or longer, Number of issues \| issues	0	3
Gross unrealized losses, 12 months or longer, Fair value	$ 0	$ 27,637
Gross unrealized losses, 12 months or longer, Amount	0	(109)
Mortgage-backed securities*
Debt Securities, Available-for-sale [Abstract]
Amortized cost	1,598,949	1,024,892
Gross unrealized gains	27,580	6,000
Gross unrealized losses	(981)	(4,507)
Estimated fair value	$ 1,625,548	$ 1,026,385
Gross unrealized losses, Less than 12 months, Number of issues \| issues	17	19
Gross unrealized losses, Less than 12 months, Fair value	$ 304,930	$ 152,071
Gross unrealized losses, Less than 12 months, Amount	$ (981)	$ (819)
Gross unrealized losses, 12 months or longer, Number of issues \| issues	0	75
Gross unrealized losses, 12 months or longer, Fair value	$ 0	$ 318,020
Gross unrealized losses, 12 months or longer, Amount	0	(3,688)
Debt Securities, Held-to-maturity [Abstract]
Amortized cost	133,858	139,451
Gross unrealized gains	4,878	4,087
Gross unrealized losses	(114)	(71)
Held-to-maturity investment securities	$ 138,622	$ 143,467
Less Than 12 Months: Number Of Issues \| issues	2	1
Less than 12 Months: Fair value	$ 28,486	$ 12,986
Less than 12 Months: Amount	$ (114)	$ (71)
12 months or longer: Number of issues \| issues	0	0
12 months or longer: Fair value	$ 0	$ 0
12 months or longer: Amount	0	0
Corporate bonds
Debt Securities, Available-for-sale [Abstract]
Amortized cost	29,772	58,694
Gross unrealized gains	1,565	1,363
Gross unrealized losses	0	0
Estimated fair value	$ 31,337	$ 60,057
Gross unrealized losses, Less than 12 months, Number of issues \| issues	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 \| issues	0	0
Gross unrealized losses, 12 months or longer, Fair value	$ 0	$ 0
Gross unrealized losses, 12 months or longer, Amount	0	0
Mortgage revenue bonds
Debt Securities, Available-for-sale [Abstract]
Amortized cost	27,185	28,597
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	$ 27,185	$ 28,597
Gross unrealized losses, Less than 12 months, Number of issues \| issues	0	0
Gross unrealized losses, Less than 12 months, Fair value		$ 0
Gross unrealized losses, Less than 12 months, Amount		$ 0
Gross unrealized losses, 12 months or longer, Number of issues \| issues	0	0
Gross unrealized losses, 12 months or longer, Fair value	$ 0	$ 0
Gross unrealized losses, 12 months or longer, Amount	$ 0	$ 0

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Debt Securities, 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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