Securities - General Disclosures (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Securities
Equity securities consisting of common stock and money market mutual funds, fair value	$ 5,690,000		$ 5,690,000	$ 6,169,000
Unrealized gains (losses) recognized on equity securities	65,000		(735,000)
Debt securities available for sale
Amortized Cost	1,675,852,000		1,675,852,000	707,213,000
Gross Unrealized Gains	25,858,000		25,858,000	798,000
Gross Unrealized Losses	(705,000)		(705,000)	(10,326,000)
Fair Value	1,701,005,000		1,701,005,000	697,685,000
Debt securities held to maturity
Amortized Cost	15,170,000		15,170,000	608,660,000
Gross Unrealized Gains				1,827,000
Gross Unrealized Losses				(7,127,000)
Fair Value	15,399,000		15,399,000	603,360,000
Debt securities available for sale, Amortized Cost
Due in one year or less	128,897,000		128,897,000
Due after one year through five years	407,251,000		407,251,000
Due after five years through ten years	248,270,000		248,270,000
Due after ten years	891,434,000		891,434,000
Amortized Cost	1,675,852,000		1,675,852,000	707,213,000
Debt securities available for sale, Fair Value
Due in one year or less	129,230,000		129,230,000
Due after one year through five years	413,491,000		413,491,000
Due after five years through ten years	253,453,000		253,453,000
Due after ten years	904,831,000		904,831,000
Fair Value	1,701,005,000		1,701,005,000	697,685,000
Debt securities held to maturity, Amortized Cost
Due in one year or less	2,215,000		2,215,000
Due after one year through five years	11,834,000		11,834,000
Due after five years through ten years	1,121,000		1,121,000
Amortized Cost	15,170,000		15,170,000	608,660,000
Debt securities held to maturity, Fair Value
Due in one year or less	2,218,000		2,218,000
Due after one year through five years	12,006,000		12,006,000
Due after five years through ten years	1,175,000		1,175,000
Fair Value	15,399,000		15,399,000	603,360,000
Realized gains and losses related to sales and calls of securities AFS
Gross security gains	298,000		689,000
Gross security (losses)	(1,000)		(585,000)
Net (losses) gains on sales of securities	297,000		104,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	59,657,000		59,657,000	190,737,000
Continuous unrealized losses existing for greater than 12 months, gross	98,208,000		98,208,000	417,336,000
Total, gross	157,865,000		157,865,000	608,073,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(116,000)		(116,000)	(1,231,000)
Continuous unrealized losses existing greater than 12 months, gross	(589,000)		(589,000)	(9,095,000)
Total, gross	$ (705,000)		$ (705,000)	(10,326,000)
Debt securities held to maturity, Fair Value
Continuous unrealized losses existing for less than 12 months, gross				98,669,000
Continuous unrealized losses existing for greater than 12 months, gross				299,707,000
Total, gross				398,376,000
Debt securities held to maturity, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross				(1,219,000)
Continuous unrealized losses existing for greater than 12 months, gross				(5,908,000)
Total, gross				(7,127,000)
Security portfolio evaluated for other-than-temporary impairment	1,208		1,208
Number of securities in unrealized loss position	74		74
Securities in unrealized loss position as a percentage of aggregate carrying value of investments	0.44%		0.44%
Collateral
Realized gains and losses related to sales and calls of securities AFS
Carrying amount of investment securities pledged as collateral	$ 768,700,000		$ 768,700,000	498,300,000
U.S. Treasury securities
Debt securities available for sale
Amortized Cost	51,395,000		51,395,000	25,824,000
Gross Unrealized Gains	262,000		262,000	1,000
Gross Unrealized Losses	(6,000)		(6,000)	(414,000)
Fair Value	51,651,000		51,651,000	25,411,000
Debt securities available for sale, Amortized Cost
Amortized Cost	51,395,000		51,395,000	25,824,000
Debt securities available for sale, Fair Value
Fair Value	51,651,000		51,651,000	25,411,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross				995,000
Continuous unrealized losses existing for greater than 12 months, gross	9,982,000		9,982,000	24,343,000
Total, gross	9,982,000		9,982,000	25,338,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross				(4,000)
Continuous unrealized losses existing greater than 12 months, gross	(6,000)		(6,000)	(410,000)
Total, gross	(6,000)		(6,000)	(414,000)
Obligations of U.S. government corporations and agencies
Debt securities available for sale
Amortized Cost	202,393,000		202,393,000	53,096,000
Gross Unrealized Gains	3,273,000		3,273,000	7,000
Gross Unrealized Losses	(53,000)		(53,000)	(761,000)
Fair Value	205,613,000		205,613,000	52,342,000
Debt securities available for sale, Amortized Cost
Amortized Cost	202,393,000		202,393,000	53,096,000
Debt securities available for sale, Fair Value
Fair Value	205,613,000		205,613,000	52,342,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	6,938,000		6,938,000	749,000
Continuous unrealized losses existing for greater than 12 months, gross	6,481,000		6,481,000	50,744,000
Total, gross	13,419,000		13,419,000	51,493,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(45,000)		(45,000)	(3,000)
Continuous unrealized losses existing greater than 12 months, gross	(8,000)		(8,000)	(758,000)
Total, gross	(53,000)		(53,000)	(761,000)
Obligations of states and political subdivisions
Debt securities available for sale
Amortized Cost	263,345,000		263,345,000	171,131,000
Gross Unrealized Gains	5,733,000		5,733,000	484,000
Gross Unrealized Losses	(16,000)		(16,000)	(1,571,000)
Fair Value	269,062,000		269,062,000	170,044,000
Debt securities held to maturity
Amortized Cost	15,170,000		15,170,000	33,947,000
Gross Unrealized Gains	229,000		229,000	68,000
Gross Unrealized Losses				(87,000)
Fair Value	15,399,000		15,399,000	33,928,000
Debt securities available for sale, Amortized Cost
Amortized Cost	263,345,000		263,345,000	171,131,000
Debt securities available for sale, Fair Value
Fair Value	269,062,000		269,062,000	170,044,000
Debt securities held to maturity, Amortized Cost
Amortized Cost	15,170,000		15,170,000	33,947,000
Debt securities held to maturity, Fair Value
Fair Value	15,399,000		15,399,000	33,928,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	4,949,000		4,949,000	49,893,000
Continuous unrealized losses existing for greater than 12 months, gross	9,129,000		9,129,000	77,651,000
Total, gross	14,078,000		14,078,000	127,544,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(11,000)		(11,000)	(460,000)
Continuous unrealized losses existing greater than 12 months, gross	(5,000)		(5,000)	(1,111,000)
Total, gross	(16,000)		(16,000)	(1,571,000)
Debt securities held to maturity, Fair Value
Continuous unrealized losses existing for less than 12 months, gross				9,531,000
Continuous unrealized losses existing for greater than 12 months, gross				9,538,000
Total, gross				19,069,000
Debt securities held to maturity, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross				(33,000)
Continuous unrealized losses existing for greater than 12 months, gross				(54,000)
Total, gross				(87,000)
Commercial mortgage-backed securities
Debt securities available for sale
Amortized Cost	121,338,000		121,338,000	2,003,000
Gross Unrealized Gains	2,248,000		2,248,000
Gross Unrealized Losses	(18,000)		(18,000)	(61,000)
Fair Value	123,568,000		123,568,000	1,942,000
Debt securities held to maturity
Amortized Cost				59,054,000
Gross Unrealized Gains				11,000
Gross Unrealized Losses				(1,003,000)
Fair Value				58,062,000
Debt securities available for sale, Amortized Cost
Amortized Cost	121,338,000		121,338,000	2,003,000
Debt securities available for sale, Fair Value
Fair Value	123,568,000		123,568,000	1,942,000
Debt securities held to maturity, Amortized Cost
Amortized Cost				59,054,000
Debt securities held to maturity, Fair Value
Fair Value				58,062,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	3,913,000		3,913,000
Continuous unrealized losses existing for greater than 12 months, gross	9,702,000		9,702,000	1,942,000
Total, gross	13,615,000		13,615,000	1,942,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(11,000)		(11,000)
Continuous unrealized losses existing greater than 12 months, gross	(7,000)		(7,000)	(61,000)
Total, gross	(18,000)		(18,000)	(61,000)
Debt securities held to maturity, Fair Value
Continuous unrealized losses existing for less than 12 months, gross				12,067,000
Continuous unrealized losses existing for greater than 12 months, gross				45,041,000
Total, gross				57,108,000
Debt securities held to maturity, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross				(212,000)
Continuous unrealized losses existing for greater than 12 months, gross				(791,000)
Total, gross				(1,003,000)
Residential mortgage-backed securities
Debt securities available for sale
Amortized Cost	914,320,000		914,320,000	322,646,000
Gross Unrealized Gains	12,674,000		12,674,000	245,000
Gross Unrealized Losses	(607,000)		(607,000)	(7,143,000)
Fair Value	926,387,000		926,387,000	315,748,000
Debt securities held to maturity
Amortized Cost				515,659,000
Gross Unrealized Gains				1,748,000
Gross Unrealized Losses				(6,037,000)
Fair Value				511,370,000
Debt securities available for sale, Amortized Cost
Amortized Cost	914,320,000		914,320,000	322,646,000
Debt securities available for sale, Fair Value
Fair Value	926,387,000		926,387,000	315,748,000
Debt securities held to maturity, Amortized Cost
Amortized Cost				515,659,000
Debt securities held to maturity, Fair Value
Fair Value				511,370,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	43,372,000		43,372,000	48,387,000
Continuous unrealized losses existing for greater than 12 months, gross	57,319,000		57,319,000	247,573,000
Total, gross	100,691,000		100,691,000	295,960,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(47,000)		(47,000)	(496,000)
Continuous unrealized losses existing greater than 12 months, gross	(560,000)		(560,000)	(6,647,000)
Total, gross	(607,000)		(607,000)	(7,143,000)
Debt securities held to maturity, Fair Value
Continuous unrealized losses existing for less than 12 months, gross				77,071,000
Continuous unrealized losses existing for greater than 12 months, gross				245,128,000
Total, gross				322,199,000
Debt securities held to maturity, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross				(974,000)
Continuous unrealized losses existing for greater than 12 months, gross				(5,063,000)
Total, gross				(6,037,000)
Corporate debt securities
Debt securities available for sale
Amortized Cost	123,061,000		123,061,000	132,513,000
Gross Unrealized Gains	1,668,000		1,668,000	61,000
Gross Unrealized Losses	(5,000)		(5,000)	(376,000)
Fair Value	124,724,000		124,724,000	132,198,000
Debt securities available for sale, Amortized Cost
Amortized Cost	123,061,000		123,061,000	132,513,000
Debt securities available for sale, Fair Value
Fair Value	124,724,000		124,724,000	132,198,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	485,000		485,000	90,713,000
Continuous unrealized losses existing for greater than 12 months, gross	5,595,000		5,595,000	15,083,000
Total, gross	6,080,000		6,080,000	105,796,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(2,000)		(2,000)	(268,000)
Continuous unrealized losses existing greater than 12 months, gross	(3,000)		(3,000)	(108,000)
Total, gross	$ (5,000)		$ (5,000)	$ (376,000)
Accounting Standards Update 2017-12
Realized gains and losses related to sales and calls of securities AFS
Transferred securities from held to maturity to available-for-sale		$ 573,600,000
Unrealized loss related to transfer of securities from held to maturity to AFS		$ 4,800,000