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	Dec. 31, 2018	Dec. 31, 2017
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	$ 278,418	$ 482,089
Past due loans of 90 days or more	0	320
Nonaccrual loans	397	741
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	267,907	467,039
Past due loans of 90 days or more	0	320
Nonaccrual loans	397	741
Difference
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage loans held for sale measured at fair value	10,511	15,050
Past due loans of 90 days or more	0	0
Nonaccrual loans	$ 0	$ 0