Investment securities - Schedule of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized cost, due in one year or less	$ 15,883	$ 905
Amortized costs, due in one to five years	13,806	28,332
Amortized cost, due in five to ten years	18,539	19,218
Amortized cost, due in over ten years	99,151	67,016
Amortized cost, total	147,379	115,471
Mortgage-backed securities - residential, amortized cost	520,654	425,557
Amortized cost	668,033	541,028
Fair value, due in one year or less	16,028	925
Fair value, due in one to five years	13,740	28,878
Fair value, due in five to ten years	18,387	19,588
Fair value, due in over ten years	98,963	68,098
Fair value, total	147,118	117,489
Mortgage-backed securities - residential, fair value	508,580	418,781
Total debt securities, Fair value	655,698	$ 536,270
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Line Items]
Amortized cost	520,654
Total debt securities, Fair value	$ 508,580