Investment securities - Schedule of Gross Unrealized Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	$ 37,163	$ 87,858
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(139)	(844)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	218,384	369,343
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(1,995)	(14,247)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	255,547	457,201
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(2,134)	(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	999	989
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(1)	(11)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	999	989
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(1)	(11)
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	26,610	60,347
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(70)	(478)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	217,385	335,769
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(1,994)	(12,787)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	243,995	396,116
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(2,064)	(13,265)
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	10,553	27,511
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(69)	(366)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	0	25,343
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	0	(1,306)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	10,553	52,854
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(69)	(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	0	7,242
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	0	(143)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	0	7,242
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 0	$ (143)