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, 2019	Dec. 31, 2018
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	$ 305,493	$ 278,418
Past due loans of 90 days or more	0	0
Nonaccrual loans	0	397
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	297,311	267,907
Past due loans of 90 days or more	0	0
Nonaccrual loans	0	397
Difference
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	8,182	10,511
Past due loans of 90 days or more	0	0
Nonaccrual loans	$ 0	$ 0