Bank segment - Collateral-dependent loans (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	$ 5,325,770	$ 5,345,226
Real estate
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	4,167,255	4,239,745
Real estate | Residential 1-4 family
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	2,107,537	2,144,239
Real estate | Commercial real estate
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	1,012,968	983,865
Real estate | Home equity line of credit
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	901,462	963,578
Real estate | Residential land
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	17,468	15,617
Commercial
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	1,010,004	936,748
Residential real estate property | Real estate | Residential 1-4 family
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	2,782	2,541
Residential real estate property | Real estate | Home equity line of credit
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	1,587	1,567
Residential real estate property | Real estate | Residential land
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	300	300
Commercial real estate | Real estate | Commercial real estate
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	2,828	2,875
Collateral Pledged
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	8,386	8,217
Collateral Pledged | Real estate
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	7,497	7,283
Business Assets | Commercial
Financing Receivable, Nonaccrual [Line Items]
Collateral-dependent loans, amortized cost	$ 889	$ 934