Securities - General Disclosures (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Debt Securities
Equity securities consisting of common stock and money market mutual funds, fair value	$ 17,808,000		$ 17,808,000		$ 5,952,000
Unrealized gains (losses) recognized on equity securities	(437,000)	$ 65,000	(1,234,000)	$ (735,000)
Debt securities available for sale
Amortized Cost	2,027,816,000		2,027,816,000		1,627,065,000
Gross Unrealized Gains	53,509,000		53,509,000		22,021,000
Gross Unrealized Losses	(476,000)		(476,000)		(829,000)
Fair Value	2,080,849,000		2,080,849,000		1,648,257,000
Debt securities available for sale, Amortized Cost
Due in one year or less	148,312,000		148,312,000
Due after one year through five years	210,254,000		210,254,000
Due after five years through ten years	260,947,000		260,947,000
Due after ten years	1,408,303,000		1,408,303,000
Amortized Cost	2,027,816,000		2,027,816,000		1,627,065,000
Debt securities available for sale, Fair Value
Due in one year or less	149,872,000		149,872,000
Due after one year through five years	219,106,000		219,106,000
Due after five years through ten years	272,794,000		272,794,000
Due after ten years	1,439,077,000		1,439,077,000
Fair Value	2,080,849,000		2,080,849,000		1,648,257,000
Realized gains and losses related to sales and calls of securities AFS
Gross security gains	11,000	298,000	1,718,000	689,000
Gross security (losses)		(1,000)	(8,000)	(585,000)
Net gains (losses) on sales of securities	11,000	$ 297,000	1,710,000	$ 104,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	217,361,000		217,361,000		122,991,000
Continuous unrealized losses existing for greater than 12 months, gross	6,317,000		6,317,000		53,851,000
Total, gross	223,678,000		223,678,000		176,842,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(418,000)		(418,000)		(448,000)
Continuous unrealized losses existing greater than 12 months, gross	(58,000)		(58,000)		(381,000)
Total, gross	$ (476,000)		$ (476,000)		(829,000)
Security portfolio evaluated for other-than-temporary impairment	1,190		1,190
Number of securities in unrealized loss position	37		37
Securities in unrealized loss position as a percentage of aggregate carrying value of investments	0.21%		0.21%
Collateral
Realized gains and losses related to sales and calls of securities AFS
Carrying amount of investment securities pledged as collateral	$ 591,700,000		$ 591,700,000		704,400,000
U.S. Treasury securities
Debt securities available for sale
Amortized Cost	29,442,000		29,442,000		51,472,000
Gross Unrealized Gains	495,000		495,000		265,000
Fair Value	29,937,000		29,937,000		51,737,000
Debt securities available for sale, Amortized Cost
Amortized Cost	29,442,000		29,442,000		51,472,000
Debt securities available for sale, Fair Value
Fair Value	29,937,000		29,937,000		51,737,000
Obligations of U.S. government corporations and agencies
Debt securities available for sale
Amortized Cost	76,021,000		76,021,000		160,364,000
Gross Unrealized Gains	2,512,000		2,512,000		2,684,000
Gross Unrealized Losses	(55,000)		(55,000)		(48,000)
Fair Value	78,478,000		78,478,000		163,000,000
Debt securities available for sale, Amortized Cost
Amortized Cost	76,021,000		76,021,000		160,364,000
Debt securities available for sale, Fair Value
Fair Value	78,478,000		78,478,000		163,000,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross					6,362,000
Continuous unrealized losses existing for greater than 12 months, gross	5,479,000		5,479,000
Total, gross	5,479,000		5,479,000		6,362,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross					(48,000)
Continuous unrealized losses existing greater than 12 months, gross	(55,000)		(55,000)
Total, gross	(55,000)		(55,000)		(48,000)
Obligations of states and political subdivisions
Debt securities available for sale
Amortized Cost	304,749,000		304,749,000		262,492,000
Gross Unrealized Gains	11,380,000		11,380,000		5,810,000
Gross Unrealized Losses	(9,000)		(9,000)		(11,000)
Fair Value	316,120,000		316,120,000		268,291,000
Debt securities available for sale, Amortized Cost
Amortized Cost	304,749,000		304,749,000		262,492,000
Debt securities available for sale, Fair Value
Fair Value	316,120,000		316,120,000		268,291,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	3,810,000		3,810,000		4,981,000
Continuous unrealized losses existing for greater than 12 months, gross					1,548,000
Total, gross	3,810,000		3,810,000		6,529,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(9,000)		(9,000)		(11,000)
Total, gross	(9,000)		(9,000)		(11,000)
Commercial mortgage-backed securities
Debt securities available for sale
Amortized Cost	346,543,000		346,543,000		137,733,000
Gross Unrealized Gains	10,804,000		10,804,000		1,700,000
Gross Unrealized Losses	(104,000)		(104,000)		(146,000)
Fair Value	357,243,000		357,243,000		139,287,000
Debt securities available for sale, Amortized Cost
Amortized Cost	346,543,000		346,543,000		137,733,000
Debt securities available for sale, Fair Value
Fair Value	357,243,000		357,243,000		139,287,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	53,690,000		53,690,000		33,322,000
Continuous unrealized losses existing for greater than 12 months, gross					2,044,000
Total, gross	53,690,000		53,690,000		35,366,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(104,000)		(104,000)		(144,000)
Continuous unrealized losses existing greater than 12 months, gross					(2,000)
Total, gross	(104,000)		(104,000)		(146,000)
Residential mortgage-backed securities
Debt securities available for sale
Amortized Cost	1,192,131,000		1,192,131,000		912,308,000
Gross Unrealized Gains	26,699,000		26,699,000		10,282,000
Gross Unrealized Losses	(304,000)		(304,000)		(624,000)
Fair Value	1,218,526,000		1,218,526,000		921,966,000
Debt securities available for sale, Amortized Cost
Amortized Cost	1,192,131,000		1,192,131,000		912,308,000
Debt securities available for sale, Fair Value
Fair Value	1,218,526,000		1,218,526,000		921,966,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	152,298,000		152,298,000		78,326,000
Continuous unrealized losses existing for greater than 12 months, gross	838,000		838,000		50,259,000
Total, gross	153,136,000		153,136,000		128,585,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(301,000)		(301,000)		(245,000)
Continuous unrealized losses existing greater than 12 months, gross	(3,000)		(3,000)		(379,000)
Total, gross	(304,000)		(304,000)		(624,000)
Corporate debt securities
Debt securities available for sale
Amortized Cost	78,930,000		78,930,000		102,696,000
Gross Unrealized Gains	1,619,000		1,619,000		1,280,000
Gross Unrealized Losses	(4,000)		(4,000)
Fair Value	80,545,000		80,545,000		103,976,000
Debt securities available for sale, Amortized Cost
Amortized Cost	78,930,000		78,930,000		102,696,000
Debt securities available for sale, Fair Value
Fair Value	80,545,000		80,545,000		$ 103,976,000
Debt securities available for sale, Fair Value
Continuous unrealized losses existing for less than 12 months, gross	7,563,000		7,563,000
Total, gross	7,563,000		7,563,000
Debt securities available for sale, Unrealized Losses
Continuous unrealized losses existing for less than 12 months, gross	(4,000)		(4,000)
Total, gross	$ (4,000)		$ (4,000)