Parent Company Financial Information (Condensed Statements of Cash Flows) (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows From Operating Activities:
Net income		$ 9,187,000	$ 10,783,000	$ 10,744,000	$ 10,841,000	$ 10,090,000	$ 10,057,000	$ 10,315,000	$ 11,111,000	$ 41,555,000	$ 41,573,000	$ 35,122,000
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income tax provision (credit)										(1,212,000)	(1,601,000)	1,114,000
Stock-based compensation expense										3,050,000	1,814,000	2,434,000
Other decreases (increases)										921,000	2,782,000	(4,212,000)
Net cash provided by operating activities										49,987,000	59,374,000	47,172,000
Cash Flows From Financing Activities:
Repurchase of common stock										(38,171,000)	(1,541,000)
Proceeds from exercise of stock options										452,000	312,000	917,000
Proceeds from issuance of common stock, net										1,420,000	17,777,000	22,988,000
Cash dividends paid										(17,249,000)	(15,585,000)	(13,703,000)
Net cash (used in) provided by financing activities										(217,724,000)	309,638,000	341,279,000
Net (decrease) increase in cash and cash equivalents										(8,390,000)	(22,314,000)	32,743,000
Cash and cash equivalents, beginning of year					47,358,000				69,672,000	47,358,000	69,672,000	36,929,000
Cash and cash equivalents, end of period		38,968,000				47,358,000				38,968,000	47,358,000	69,672,000
Supplemental Schedule of Noncash Financing Activities:
Cash dividends payable		4,308,000				4,456,000				4,308,000	4,456,000	3,798,000
Parent Company [Member]
Cash Flows From Operating Activities:
Net income										41,555,000	41,573,000	35,122,000
Adjustments to reconcile net income to net cash provided by operating activities:
Dividends in excess of earnings of subsidiary bank										11,458,000
Undistributed earnings of subsidiary bank										11,458,000	(26,763,000)	(25,577,000)
Deferred income tax provision (credit)										7,000	(543,000)	167,000
Stock-based compensation expense										3,050,000	1,814,000	2,434,000
Increase (decrease) in other liabilities										(119,000)	307,000	5,000
Other decreases (increases)										(815,000)	1,619,000	(1,383,000)
Net cash provided by operating activities										55,136,000	18,007,000	10,768,000
Cash Flows From Investing Activities:
Capital contributions to Bank subsidiary											(19,000,000)	(20,500,000)
Cash Flows From Financing Activities:
Repurchase of common stock										(38,171,000)	(1,541,000)
Shares withheld upon the vesting and conversion of RSUs										(854,000)	(774,000)	(527,000)
Proceeds from exercise of stock options										452,000	312,000	917,000
Proceeds from issuance of common stock, net										1,822,000	18,239,000	22,598,000
Cash dividends paid										(17,249,000)	(15,585,000)	(13,703,000)
Net cash (used in) provided by financing activities										(54,000,000)	651,000	9,285,000
Net (decrease) increase in cash and cash equivalents	[1]									1,136,000	(342,000)	(447,000)
Cash and cash equivalents, beginning of year					$ 583,000				$ 925,000	583,000	925,000	1,372,000
Cash and cash equivalents, end of period		1,719,000				583,000				1,719,000	583,000	925,000
Supplemental Schedule of Noncash Financing Activities:
Cash dividends payable		$ 4,308,000				$ 4,456,000				$ 4,308,000	$ 4,456,000	$ 3,798,000

[1] Cash and cash equivalents is defined as cash and due from banks and includes, among other things, the checking and money market accounts with the Corporation’s wholly-owned bank subsidiary.