Investment securities - Schedule of Gross Unrealized Losses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	$ 87,858
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(844)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	369,343
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(14,247)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	457,201
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(15,091)
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		$ 122,217
Available-for-sale securities, continuous unrealized loss position, less than twelve months		(1,174)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value		314,406
Available-for-sale securities, continuous unrealized loss position, twelve months or longer		(6,799)
Available-for-sale securities, continuous unrealized loss position, fair value		436,623
Available-for-sale securities, continuous unrealized loss position,		(7,973)
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
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	0
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	989
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(11)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	989
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(11)
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		0
Available-for-sale securities, continuous unrealized loss position, less than twelve months		0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value		986
Available-for-sale securities, continuous unrealized loss position, twelve months or longer		(13)
Available-for-sale securities, continuous unrealized loss position, fair value		986
Available-for-sale securities, continuous unrealized loss position,		(13)
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	60,347
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(478)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	335,769
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(12,787)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	396,116
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(13,265)
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		107,611
Available-for-sale securities, continuous unrealized loss position, less than twelve months		(980)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value		290,258
Available-for-sale securities, continuous unrealized loss position, twelve months or longer		(6,170)
Available-for-sale securities, continuous unrealized loss position, fair value		397,869
Available-for-sale securities, continuous unrealized loss position,		(7,150)
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	27,511
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(366)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	25,343
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(1,306)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	52,854
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(1,672)
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		7,354
Available-for-sale securities, continuous unrealized loss position, less than twelve months		(101)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value		20,112
Available-for-sale securities, continuous unrealized loss position, twelve months or longer		(467)
Available-for-sale securities, continuous unrealized loss position, fair value		27,466
Available-for-sale securities, continuous unrealized loss position,		(568)
Treasury securities
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	0
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	0
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	7,242
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	(143)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	7,242
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ (143)
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		7,252
Available-for-sale securities, continuous unrealized loss position, less than twelve months		(93)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value		0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer		0
Available-for-sale securities, continuous unrealized loss position, fair value		7,252
Available-for-sale securities, continuous unrealized loss position,		(93)
Debt Securities
Debt Securities, Available-for-sale [Abstract]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		122,217
Available-for-sale securities, continuous unrealized loss position, less than twelve months		(1,174)
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value		311,356
Available-for-sale securities, continuous unrealized loss position, twelve months or longer		(6,650)
Available-for-sale securities, continuous unrealized loss position, fair value		433,573
Available-for-sale securities, continuous unrealized loss position,		(7,824)
Equity securities
Debt Securities, Available-for-sale [Abstract]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		0
Available-for-sale securities, continuous unrealized loss position, less than twelve months		0
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value		3,050
Available-for-sale securities, continuous unrealized loss position, twelve months or longer		(149)
Available-for-sale securities, continuous unrealized loss position, fair value		3,050
Available-for-sale securities, continuous unrealized loss position,		$ (149)