Bank segment - Collateral-dependent loans (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	$ 5,494,110	$ 5,120,664
Real estate
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	4,261,537	4,192,069
Real estate | Residential 1-4 family
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	2,195,093	2,178,135
Real estate | Home equity line of credit
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	1,028,011	1,092,125
Real estate | Commercial construction
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	112,930	70,605
Commercial
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	1,042,435	$ 670,674
Residential real estate property | Real estate | Residential 1-4 family
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	2,057
Residential real estate property | Real estate | Home equity line of credit
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	1,555
Business Assets | Real estate | Commercial construction
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	0
Collateral Pledged
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	3,612
Collateral Pledged | Real estate
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	3,612
Commercial real estate | Commercial
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	$ 0