Fair value of financial instruments - Schedule of Differences between Fair Value and Principal Balance for Loans Held for Sale Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	$ 610,695	$ 262,518
Past due loans of 90 days or more	0	0
Nonaccrual loans	12,812	0
Fair Value | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	228,444
Aggregate Unpaid Principal Balance
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	587,352	254,868
Past due loans of 90 days or more	0	0
Nonaccrual loans	12,812	0
Aggregate Unpaid Principal Balance | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	250,649
Difference
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	23,343	7,650
Past due loans of 90 days or more	0	0
Nonaccrual loans	0	$ 0
Difference | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	$ (22,205)