Bank segment - Components of investment securities (Details) $ in Thousands	Sep. 30, 2018 USD ($) issue	Dec. 31, 2017 USD ($) issue
Debt Securities, Available-for-sale [Abstract]
Amortized cost	$ 1,439,098	$ 1,421,622
Gross unrealized gains	362	1,231
Gross unrealized losses	(51,889)	(21,655)
Estimated fair value	$ 1,387,571	$ 1,401,198
Gross unrealized losses, Less than 12 months, Number of issues \| issue	75	82
Gross unrealized losses, Less than 12 months, Fair value	$ 563,811	$ 736,772
Gross unrealized losses, Less than 12 months, Amount	$ (15,376)	$ (7,664)
Gross unrealized losses, 12 months or longer, Number of issues \| issue	128	85
Gross unrealized losses, 12 months or longer, Fair value	$ 759,281	$ 522,208
Gross unrealized losses, 12 months or longer, Amount	(36,513)	(13,991)
Debt Securities, Held-to-maturity [Abstract]
Amortized cost	102,498	44,515
Gross unrealized gains	0	1
Gross unrealized losses	(2,569)	(104)
Held-to-maturity investment securities	$ 99,929	$ 44,412
Less Than 12 Months: Number Of Issues \| issue	7	2
Less than 12 Months: Fair value	$ 99,929	$ 35,744
Less than 12 Months: Amount	$ (2,569)	$ (104)
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
Debt Securities, Available-for-sale [Abstract]
Amortized cost	175,144	185,891
Gross unrealized gains	24	438
Gross unrealized losses	(4,754)	(2,031)
Estimated fair value	$ 170,414	$ 184,298
Gross unrealized losses, Less than 12 months, Number of issues \| issue	11	15
Gross unrealized losses, Less than 12 months, Fair value	$ 67,258	$ 83,137
Gross unrealized losses, Less than 12 months, Amount	$ (1,339)	$ (825)
Gross unrealized losses, 12 months or longer, Number of issues \| issue	17	8
Gross unrealized losses, 12 months or longer, Fair value	$ 93,132	$ 62,296
Gross unrealized losses, 12 months or longer, Amount	(3,415)	(1,206)
Mortgage-related securities- FNMA, FHLMC and GNMA
Debt Securities, Available-for-sale [Abstract]
Amortized cost	1,195,492	1,220,304
Gross unrealized gains	292	793
Gross unrealized losses	(47,094)	(19,624)
Estimated fair value	$ 1,148,690	$ 1,201,473
Gross unrealized losses, Less than 12 months, Number of issues \| issue	59	67
Gross unrealized losses, Less than 12 months, Fair value	$ 473,714	$ 653,635
Gross unrealized losses, Less than 12 months, Amount	$ (13,996)	$ (6,839)
Gross unrealized losses, 12 months or longer, Number of issues \| issue	111	77
Gross unrealized losses, 12 months or longer, Fair value	$ 666,149	$ 459,912
Gross unrealized losses, 12 months or longer, Amount	(33,098)	(12,785)
Debt Securities, Held-to-maturity [Abstract]
Amortized cost	102,498	44,515
Gross unrealized gains	0	1
Gross unrealized losses	(2,569)	(104)
Held-to-maturity investment securities	$ 99,929	$ 44,412
Less Than 12 Months: Number Of Issues \| issue	7	2
Less than 12 Months: Fair value	$ 99,929	$ 35,744
Less than 12 Months: Amount	$ (2,569)	$ (104)
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
Debt Securities, Available-for-sale [Abstract]
Amortized cost	49,378
Gross unrealized gains	46
Gross unrealized losses	(41)
Estimated fair value	$ 49,383
Gross unrealized losses, Less than 12 months, Number of issues \| issue	5
Gross unrealized losses, Less than 12 months, Fair value	$ 22,839
Gross unrealized losses, Less than 12 months, Amount	$ (41)
Gross unrealized losses, 12 months or longer, Number of issues \| issue	0
Gross unrealized losses, 12 months or longer, Fair value	$ 0
Gross unrealized losses, 12 months or longer, Amount	0
Mortgage revenue bonds
Debt Securities, Available-for-sale [Abstract]
Amortized cost	19,084	15,427
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	$ 19,084	$ 15,427
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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