Bank segment - Components of investment securities (Details) $ in Thousands	Mar. 31, 2019 USD ($) issue	Dec. 31, 2018 USD ($) issue
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 1,368,733	$ 1,421,897
Gross unrealized gains	2,456	954
Gross unrealized losses	(22,926)	(34,318)
Estimated fair value	$ 1,348,263	$ 1,388,533
Gross unrealized losses, Less than 12 months, Number of issues \| issue	5	33
Gross unrealized losses, Less than 12 months, Fair value	$ 23,814	$ 178,068
Gross unrealized losses, Less than 12 months, Amount	$ (17)	$ (1,907)
Gross unrealized losses, 12 months or longer, Number of issues \| issue	181	164
Gross unrealized losses, 12 months or longer, Fair value	$ 1,127,667	$ 1,066,295
Gross unrealized losses, 12 months or longer, Amount	(22,909)	(32,411)
Debt Securities, Held-to-maturity [Abstract]
Amortized cost	140,203	141,875
Gross unrealized gains	2,528	1,446
Gross unrealized losses	(398)	(1,264)
Held-to-maturity investment securities	$ 142,333	$ 142,057
Less Than 12 Months: Number Of Issues \| issue	0	3
Less than 12 Months: Fair value	$ 0	$ 29,814
Less than 12 Months: Amount	$ 0	$ (400)
12 months or longer: Number of issues \| issue	3	2
12 months or longer: Fair value	$ 39,027	$ 31,505
12 months or longer: Amount	(398)	(864)
U.S. Treasury and federal agency obligations
Debt Securities, Available-for-sale [Abstract]
Amortized cost	142,179	156,694
Gross unrealized gains	93	62
Gross unrealized losses	(1,428)	(2,407)
Estimated fair value	$ 140,844	$ 154,349
Gross unrealized losses, Less than 12 months, Number of issues \| issue	2	5
Gross unrealized losses, Less than 12 months, Fair value	$ 10,022	$ 25,882
Gross unrealized losses, Less than 12 months, Amount	$ (7)	$ (208)
Gross unrealized losses, 12 months or longer, Number of issues \| issue	20	19
Gross unrealized losses, 12 months or longer, Fair value	$ 117,499	$ 118,405
Gross unrealized losses, 12 months or longer, Amount	(1,421)	(2,199)
Mortgage-backed securities — issued or guaranteed by U.S. Government agencies or sponsored agencies
Debt Securities, Available-for-sale [Abstract]
Amortized cost	1,149,167	1,192,169
Gross unrealized gains	1,318	789
Gross unrealized losses	(21,498)	(31,542)
Estimated fair value	$ 1,128,987	$ 1,161,416
Gross unrealized losses, Less than 12 months, Number of issues \| issue	3	22
Gross unrealized losses, Less than 12 months, Fair value	$ 13,792	$ 129,011
Gross unrealized losses, Less than 12 months, Amount	$ (10)	$ (1,330)
Gross unrealized losses, 12 months or longer, Number of issues \| issue	161	145
Gross unrealized losses, 12 months or longer, Fair value	$ 1,010,168	$ 947,890
Gross unrealized losses, 12 months or longer, Amount	(21,488)	(30,212)
Debt Securities, Held-to-maturity [Abstract]
Amortized cost	140,203	141,875
Gross unrealized gains	2,528	1,446
Gross unrealized losses	(398)	(1,264)
Held-to-maturity investment securities	$ 142,333	$ 142,057
Less Than 12 Months: Number Of Issues \| issue	0	3
Less than 12 Months: Fair value	$ 0	$ 29,814
Less than 12 Months: Amount	$ 0	$ (400)
12 months or longer: Number of issues \| issue	3	2
12 months or longer: Fair value	$ 39,027	$ 31,505
12 months or longer: Amount	(398)	(864)
Corporate bonds
Debt Securities, Available-for-sale [Abstract]
Amortized cost	49,417	49,398
Gross unrealized gains	1,045	103
Gross unrealized losses	0	(369)
Estimated fair value	$ 50,462	$ 49,132
Gross unrealized losses, Less than 12 months, Number of issues \| issue	0	6
Gross unrealized losses, Less than 12 months, Fair value	$ 0	$ 23,175
Gross unrealized losses, Less than 12 months, Amount	$ 0	$ (369)
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 longer, Amount	0	0
Mortgage revenue bonds
Debt Securities, Available-for-sale [Abstract]
Amortized cost	27,970	23,636
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	$ 27,970	$ 23,636
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 longer, Amount	$ 0	$ 0

Debt Securities, Held-To-Maturity Securities, Less Than 12 Months, Number Of Issues

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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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