Securities - General Disclosures (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2019		Mar. 31, 2019	Jun. 30, 2019		Dec. 31, 2018
Securities
Equity securities consisting of common stock and money market mutual funds, fair value	$ 5,362,000			$ 5,362,000		$ 6,169,000
Unrealized (losses) gains recognized on equity securities	(1,016,000)			(800,000)
Debt securities available for sale
Amortized Cost	1,827,614,000			1,827,614,000		707,213,000
Gross Unrealized Gains	21,899,000			21,899,000		798,000
Gross Unrealized Losses	(1,440,000)			(1,440,000)		(10,326,000)
Fair Value	1,848,073,000			1,848,073,000		697,685,000
Debt securities held to maturity
Amortized Cost	15,708,000			15,708,000		608,660,000
Gross Unrealized Gains						1,827,000
Gross Unrealized Losses						(7,127,000)
Fair Value	15,934,000			15,934,000		603,360,000
Debt securities available for sale, Amortized Cost
Due in one year or less	220,023,000			220,023,000
Due after one year through five years	418,649,000			418,649,000
Due after five years through ten years	263,388,000			263,388,000
Due after ten years	925,554,000			925,554,000
Amortized Cost	1,827,614,000			1,827,614,000		707,213,000
Debt securities available for sale, Fair Value
Due in one year or less	220,304,000			220,304,000
Due after one year through five years	424,241,000			424,241,000
Due after five years through ten years	267,989,000			267,989,000
Due after ten years	935,539,000			935,539,000
Fair Value	1,848,073,000			1,848,073,000		697,685,000
Debt securities held to maturity, Amortized Cost
Due in one year or less	2,724,000			2,724,000
Due after one year through five years	11,861,000			11,861,000
Due after five years through ten years	1,123,000			1,123,000
Amortized Cost	15,708,000			15,708,000		608,660,000
Debt securities held to maturity, Fair Value
Due in one year or less	2,729,000			2,729,000
Due after one year through five years	12,035,000			12,035,000
Due after five years through ten years	1,170,000			1,170,000
Fair Value	15,934,000			15,934,000		603,360,000
Realized gains and losses related to sales and calls of securities AFS
Gross security gains	391,000			391,000
Gross security (losses)	(401,000)			(584,000)
Security gains (losses), net	(10,000)			(193,000)
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	16,839,000			16,839,000		190,737,000
Continuous unrealized losses existing for greater than 12 months, gross	284,775,000			284,775,000		417,336,000
Total, gross	301,614,000			301,614,000		608,073,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(23,000)			(23,000)		(1,231,000)
Continuous unrealized losses existing greater than 12 months, gross	(1,417,000)			(1,417,000)		(9,095,000)
Total, gross	$ (1,440,000)			$ (1,440,000)		(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	1,273			1,273
Number of securities in unrealized loss position	126			126
Securities in unrealized loss position as a percentage of aggregate carrying value of investments	0.47%			0.47%
Collateral
Realized gains and losses related to sales and calls of securities AFS
Carrying amount of investment securities pledged as collateral	$ 755,100,000			$ 755,100,000		498,300,000
U.S. Treasury securities
Debt securities available for sale
Amortized Cost	58,308,000			58,308,000		25,824,000
Gross Unrealized Gains	260,000			260,000		1,000
Gross Unrealized Losses	(33,000)			(33,000)		(414,000)
Fair Value	58,535,000			58,535,000		25,411,000
Debt securities available for sale, Amortized Cost
Amortized Cost	58,308,000			58,308,000		25,824,000
Debt securities available for sale, Fair Value
Fair Value	58,535,000			58,535,000		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	24,768,000			24,768,000		24,343,000
Total, gross	24,768,000			24,768,000		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	(33,000)			(33,000)		(410,000)
Total, gross	(33,000)			(33,000)		(414,000)
Obligations of U.S. government corporations and agencies
Debt securities available for sale
Amortized Cost	287,444,000			287,444,000		53,096,000
Gross Unrealized Gains	3,370,000			3,370,000		7,000
Gross Unrealized Losses	(48,000)			(48,000)		(761,000)
Fair Value	290,766,000			290,766,000		52,342,000
Debt securities available for sale, Amortized Cost
Amortized Cost	287,444,000			287,444,000		53,096,000
Debt securities available for sale, Fair Value
Fair Value	290,766,000			290,766,000		52,342,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	51,000			51,000		749,000
Continuous unrealized losses existing for greater than 12 months, gross	22,468,000			22,468,000		50,744,000
Total, gross	22,519,000			22,519,000		51,493,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross						(3,000)
Continuous unrealized losses existing greater than 12 months, gross	(48,000)			(48,000)		(758,000)
Total, gross	(48,000)			(48,000)		(761,000)
Obligations of states and political subdivisions
Debt securities available for sale
Amortized Cost	274,301,000			274,301,000		171,131,000
Gross Unrealized Gains	4,683,000			4,683,000		484,000
Gross Unrealized Losses	(54,000)			(54,000)		(1,571,000)
Fair Value	278,930,000			278,930,000		170,044,000
Debt securities held to maturity
Amortized Cost	15,708,000	[1]		15,708,000	[1]	33,947,000
Gross Unrealized Gains	226,000	[1]		226,000	[1]	68,000
Gross Unrealized Losses						(87,000)
Fair Value	15,934,000	[1]		15,934,000	[1]	33,928,000
Debt securities available for sale, Amortized Cost
Amortized Cost	274,301,000			274,301,000		171,131,000
Debt securities available for sale, Fair Value
Fair Value	278,930,000			278,930,000		170,044,000
Debt securities held to maturity, Amortized Cost
Amortized Cost	15,708,000	[1]		15,708,000	[1]	33,947,000
Debt securities held to maturity, Fair Value
Fair Value	15,934,000	[1]		15,934,000	[1]	33,928,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	1,394,000			1,394,000		49,893,000
Continuous unrealized losses existing for greater than 12 months, gross	24,192,000			24,192,000		77,651,000
Total, gross	25,586,000			25,586,000		127,544,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(8,000)			(8,000)		(460,000)
Continuous unrealized losses existing greater than 12 months, gross	(46,000)			(46,000)		(1,111,000)
Total, gross	(54,000)			(54,000)		(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	500,000			500,000		9,538,000
Total, gross	500,000			500,000		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	122,510,000			122,510,000		2,003,000
Gross Unrealized Gains	1,678,000			1,678,000
Gross Unrealized Losses	(33,000)			(33,000)		(61,000)
Fair Value	124,155,000			124,155,000		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	122,510,000			122,510,000		2,003,000
Debt securities available for sale, Fair Value
Fair Value	124,155,000			124,155,000		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 greater than 12 months, gross	14,746,000			14,746,000		1,942,000
Total, gross	14,746,000			14,746,000		1,942,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing greater than 12 months, gross	(33,000)			(33,000)		(61,000)
Total, gross	(33,000)			(33,000)		(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	951,998,000			951,998,000		322,646,000
Gross Unrealized Gains	10,394,000			10,394,000		245,000
Gross Unrealized Losses	(1,255,000)			(1,255,000)		(7,143,000)
Fair Value	961,137,000			961,137,000		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	951,998,000			951,998,000		322,646,000
Debt securities available for sale, Fair Value
Fair Value	961,137,000			961,137,000		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	13,629,000			13,629,000		48,387,000
Continuous unrealized losses existing for greater than 12 months, gross	183,552,000			183,552,000		247,573,000
Total, gross	197,181,000			197,181,000		295,960,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(8,000)			(8,000)		(496,000)
Continuous unrealized losses existing greater than 12 months, gross	(1,247,000)			(1,247,000)		(6,647,000)
Total, gross	(1,255,000)			(1,255,000)		(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	133,053,000			133,053,000		132,513,000
Gross Unrealized Gains	1,514,000			1,514,000		61,000
Gross Unrealized Losses	(17,000)			(17,000)		(376,000)
Fair Value	134,550,000			134,550,000		132,198,000
Debt securities available for sale, Amortized Cost
Amortized Cost	133,053,000			133,053,000		132,513,000
Debt securities available for sale, Fair Value
Fair Value	134,550,000			134,550,000		132,198,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	1,765,000			1,765,000		90,713,000
Continuous unrealized losses existing for greater than 12 months, gross	15,049,000			15,049,000		15,083,000
Total, gross	16,814,000			16,814,000		105,796,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(7,000)			(7,000)		(268,000)
Continuous unrealized losses existing greater than 12 months, gross	(10,000)			(10,000)		(108,000)
Total, gross	$ (17,000)			$ (17,000)		$ (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

[1] Gross unrealized losses less than one thousand dollars.