Bank segment - Loans receivable (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss	$ 5,494,110		$ 5,120,664
Deferred fees and discounts	(13,208)		512
Allowance for credit losses	(91,459)	$ (81,307)	(53,355)	$ (53,040)	$ (58,425)	$ (52,119)
Total financing receivables	$ 5,389,443		5,067,821
Minimum benchmark percentage of loan to appraisal ratio which mortgage insurance is required	80.00%
Real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss	$ 4,261,537		4,192,069
Real estate | Residential 1-4 family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss	2,195,093		2,178,135
Allowance for credit losses	(3,637)	(3,911)	(2,380)	(1,979)	(2,015)	(1,976)
Real estate | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss	900,912		824,830
Allowance for credit losses	(32,149)	(21,100)	(15,053)	(15,415)	(15,811)	(14,505)
Total financing receivables			824,830
Real estate | Home equity line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,028,011		1,092,125
Allowance for credit losses	(5,874)	(6,214)	(6,922)	(7,007)	(6,881)	(6,371)
Real estate | Residential land
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss	14,310		14,704
Allowance for credit losses	(546)	(356)	(449)	(461)	(537)	(479)
Real estate | Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss	112,930		70,605
Allowance for credit losses	(6,039)	(4,757)	(2,097)	(2,242)	(2,046)	(2,790)
Total financing receivables			70,605
Real estate | Residential construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss	10,281		11,670
Allowance for credit losses	(11)	(14)	(3)	(3)	(2)	(4)
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss	1,042,435		670,674
Allowance for credit losses	(18,192)	(13,868)	(10,245)	(8,382)	(13,073)	(9,225)
Total financing receivables			670,674
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, before Allowance for Credit Loss	190,138		257,921
Allowance for credit losses	$ (25,011)	$ (31,087)	$ (16,206)	$ (17,551)	$ (18,060)	$ (16,769)