Parent Company Cash Flow Statement (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net income		$ 29,934	$ 33,015	$ 31,118	$ 27,555	$ 22,064	$ 23,852	$ 20,563	$ 20,725	$ 121,622	$ 87,204	$ 76,648
Adjustments to reconcile net income to net cash provided by operating activities
Deferred income tax expense										(4,497)	9,211	(5)
Change in other liabilities										7,100	(3,825)	(8,738)
Net cash provided by operating activities										141,837	130,909	92,899
Cash flows used in investing activities
Net cash used in investing activities										(387,511)	(316,769)	(238,886)
Cash flows provided by financing activities
Restricted stock awards issued, net of awards surrendered										(1,371)	(1,422)	(696)
Cash received from stock option exercises										184	214	201
Proceeds from shares issued under the direct stock purchase plan										2,712	1,636	2,323
Common dividends paid										(40,167)	(34,045)	(29,711)
Net cash provided by financing activities										283,013	109,881	159,317
Net increase (decrease) in cash and cash equivalents										37,339	(75,979)	13,330
Cash and cash equivalents at beginning of year					213,116				289,095	213,116	289,095	275,765
Cash and cash equivalents at end of period		250,455				213,116				250,455	213,116	289,095
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Investments in subsidiaries	[1]	1,128,407				1,010,125				1,128,407	1,010,125
Net income										121,622	87,204	76,648
Adjustments to reconcile net income to net cash provided by operating activities
Amortization (accretion)										54	12	(154)
Deferred income tax expense										49	51	678
Change in prepaid income taxes and other assets										135	(99)	423
Change in other liabilities										6	(562)	(5,532)
Equity in undistributed income of subsidiaries										(53,450)	(42,620)	(36,062)
Net cash provided by operating activities										68,416	43,986	36,001
Cash flows used in investing activities
Cash paid for acquisitions, net of cash acquired (1)	[2]									(13,649)	(4,834)	(950)
Net cash used in investing activities										(13,649)	(4,834)	(950)
Cash flows provided by financing activities
Restricted stock awards issued, net of awards surrendered										(1,371)	(1,422)	(696)
Cash received from stock option exercises										184	214	201
Proceeds from shares issued under the direct stock purchase plan										2,712	1,636	2,323
Common dividends paid										(40,167)	(34,045)	(29,711)
Net cash provided by financing activities										(38,642)	(33,617)	(27,883)
Net increase (decrease) in cash and cash equivalents										16,125	5,535	7,168
Cash and cash equivalents at beginning of year					$ 48,131				$ 42,596	48,131	42,596	35,428
Cash and cash equivalents at end of period		64,256				48,131				64,256	48,131	$ 42,596
Cash	[3]	$ 64,256				$ 48,131				$ 64,256	$ 48,131

[1] eliminates in consolidation

[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, 2018, 2017, and 2016, respectively.

[3] Entire balance eliminates in consolidation.