Investment securities - Schedule of Gross Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	$ 4,913	$ 87,858
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(10)	(844)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	387,308	369,343
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(8,100)	(14,247)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	392,221	457,201
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(8,110)	(15,091)
U.S. government agency securities
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	0	0
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	0	0
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	993	989
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(7)	(11)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	993	989
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(7)	(11)
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	4,913	60,347
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(10)	(478)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	357,430	335,769
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(7,598)	(12,787)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	362,343	396,116
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(7,608)	(13,265)
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	0	27,511
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	0	(366)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	21,580	25,343
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(406)	(1,306)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	21,580	52,854
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(406)	(1,672)
Treasury securities
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	0	0
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	0	0
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	7,305	7,242
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(89)	(143)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	7,305	7,242
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ (89)	$ (143)