PARENT COMPANY FINANCIALS ONLY (Details 2) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Dividends received from subsidiaries(1)										$ (46,900,000)	$ (44,500,000)
Net income		$ 22,064,000	$ 23,852,000	$ 20,563,000	$ 20,725,000	$ 17,179,000	$ 20,484,000	$ 20,374,000	$ 18,611,000	87,204,000	76,648,000	$ 64,960,000
Adjustments to reconcile net income to net cash provided by operating activities
Deferred income tax expense										9,211,000	(5,000)	10,220,000
Change in other assets										20,022,000	7,627,000	3,842,000
Change in other liabilities										(3,825,000)	(8,738,000)	(2,450,000)
Net cash provided by operating activities										130,909,000	93,336,000	89,543,000
Cash flows used in investing activities
Net cash used in investing activities										(316,769,000)	(238,886,000)	(205,541,000)
Cash flows provided by financing activities
Restricted stock awards issued, net of awards surrendered										(1,422,000)	(696,000)	(657,000)
Cash received from stock option exercises										214,000	201,000	1,367,000
Proceeds from shares issued under the direct stock purchase plan										1,636,000	2,323,000	2,695,000
Common dividends paid										(34,045,000)	(29,711,000)	(26,172,000)
Net cash provided by financing activities										109,881,000	158,880,000	213,509,000
Net increase (decrease) in cash and cash equivalents										(75,979,000)	13,330,000	97,511,000
Cash and cash equivalents at beginning of year					289,095,000				275,765,000	289,095,000	275,765,000	178,254,000
Cash and cash equivalents at end of period		213,116,000				289,095,000				213,116,000	289,095,000	275,765,000
Consolidation, Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Dividends received from subsidiaries(1)										(67,000)	(62,000)	(55,000)
Investments in subsidiaries		1,000,000,000				933,600,000				1,000,000,000	933,600,000
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Investments in subsidiaries	[1]	1,010,125,000				935,778,000				1,010,125,000	935,778,000
Net income										87,204,000	76,648,000	64,960,000
Adjustments to reconcile net income to net cash provided by operating activities
Amortization (accretion)										12,000	(154,000)	(150,000)
Deferred income tax expense										51,000	678,000	3,266,000
Change in other assets										(99,000)	423,000	7,488,000
Change in other liabilities										(562,000)	(5,532,000)	(254,000)
Equity in undistributed income of subsidiaries										(42,620,000)	(36,062,000)	(30,226,000)
Net cash provided by operating activities										43,986,000	36,001,000	45,084,000
Cash flows used in investing activities
Cash paid for acquisitions, net of cash acquired (1)	[2]									(4,834,000)	(950,000)	(51,680,000)
Net cash used in investing activities										(4,834,000)	(950,000)	(51,680,000)
Cash flows provided by financing activities
Restricted stock awards issued, net of awards surrendered										(1,422,000)	(696,000)	(657,000)
Cash received from stock option exercises										214,000	201,000	1,367,000
Proceeds from shares issued under the direct stock purchase plan										1,636,000	2,323,000	2,695,000
Common dividends paid										(34,045,000)	(29,711,000)	(26,172,000)
Net cash provided by financing activities										(33,617,000)	(27,883,000)	(22,767,000)
Net increase (decrease) in cash and cash equivalents										5,535,000	7,168,000	(29,363,000)
Cash and cash equivalents at beginning of year					$ 42,596,000				$ 35,428,000	42,596,000	35,428,000	64,791,000
Cash and cash equivalents at end of period		48,131,000				42,596,000				48,131,000	42,596,000	$ 35,428,000
Cash	[3]	$ 48,131,000				$ 42,596,000				$ 48,131,000	$ 42,596,000

[1] $1.0 billion and $933.6 million eliminate in consolidation at December 31, 2017 and 2016, respectively.

[2] The majority of the net assets acquired at the parent company represented each of the acquired companies' investments in their wholly owned subsidiaries, which were eliminated in consolidation at December 31, 2017, 2016, and 2015, respectively.

[3] Entire balance eliminates in consolidation.