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	Mar. 31, 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	$ 325,304	$ 262,518
Past due loans of 90 days or more	0	0
Nonaccrual loans	0	0
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	311,836	254,868
Past due loans of 90 days or more	0	0
Nonaccrual loans	0	0
Difference
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	13,468	7,650
Past due loans of 90 days or more	0	0
Nonaccrual loans	$ 0	$ 0