PARENT COMPANY STATEMENTS - CONDENSED STATEMENTS OF CASH FLOWS (Details) - USD ($)		3 Months Ended								12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income		$ 24,592,000	$ 22,776,000	$ 19,670,000	$ 17,299,000	$ 22,198,000	$ 21,454,000	$ 21,713,000	$ 21,682,000	$ 84,337,000	$ 87,047,000	$ 80,411,000
Adjustments to net cash from operating activities:
Net cash from operating activities										87,228,000	100,039,000	104,975,000
Cash flows from financing activities:
Purchase of treasury stock	$ (928,000)									(10,547,000)	(515,000)	(463,000)
Sales of treasury stock										119,000	118,000	115,000
Net cash from financing activities										775,342,000	(48,887,000)	102,209,000
Net increase (decrease) in cash and cash equivalents										150,546,000	(117,541,000)	40,742,000
Cash and cash equivalents at beginning of the year					99,381,000				216,922,000	99,381,000	216,922,000	176,180,000
Cash and cash equivalents at end of the year	99,381,000	249,927,000				99,381,000				249,927,000	99,381,000	216,922,000
Reportable Legal Entities | Parent Company
Cash flows from operating activities:
Net income										84,337,000	87,047,000	80,411,000
Adjustments to net cash from operating activities:
Equity in undistributed income of subsidiaries										(53,828,000)	(32,533,000)	(56,567,000)
Other changes										1,257,000	3,529,000	7,294,000
Net cash from operating activities										31,766,000	58,043,000	31,138,000
Cash flows from financing activities:
Repayment of long-term debt											(30,928,000)
Proceeds from short-term borrowings										10,500,000
Payments related to equity incentive plans										(2,137,000)	(2,109,000)	(2,435,000)
Purchase of treasury stock										(10,547,000)	(515,000)	(463,000)
Sales of treasury stock										119,000	118,000	115,000
Dividends paid										(30,566,000)	(29,639,000)	(25,278,000)
Net cash from financing activities										(32,631,000)	(63,073,000)	(28,061,000)
Net increase (decrease) in cash and cash equivalents										(865,000)	(5,030,000)	3,077,000
Cash and cash equivalents at beginning of the year					$ 3,866,000				$ 8,896,000	3,866,000	8,896,000	5,819,000
Cash and cash equivalents at end of the year	$ 3,866,000	$ 3,001,000				$ 3,866,000				$ 3,001,000	$ 3,866,000	$ 8,896,000