Condensed Financial Information (parent company only) - Statement of Cash Flows (Details) - USD ($) $ in Thousands	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
Cash Flow From Operating Activities:
Net income	$ 24,383	$ 24,358	$ 24,136	$ 23,193	$ 22,293	$ 21,059	$ 20,006	$ 17,693	$ 96,070	$ 81,051	$ 65,384
Adjustments to Reconcile Net Income to Net Cash:
Share-based compensation									2,827	2,600	2,270
Gain on sale of insurance subsidiary											(8,265)
Gain on cancellation of subordinated debentures											(1,250)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments									(2,631)	(3,389)	(2,670)
Net Change in:
Net cash provided by operating activities									126,502	103,719	56,660
Cash Flows from Investing Activities:
Purchase of securities available for sale									(479,045)	(281,470)	(200,154)
Net cash received (paid) in acquisition									54,536		(7,936)
Other									(4,047)	(4,490)	9
Net cash used in investing activities									(635,330)	(448,704)	(340,391)
Cash Flows from Financing Activities:
Repayment of borrowings									(1,024,166)	(982,449)	(370,813)
Stock issued under employee benefit plans									519	462	460
Stock issued under dividend reinvestment and stock purchase plan									991	835	661
Stock options exercised									2,398	2,602	1,531
Stock redeemed									(1,283)	(1,980)	(1,640)
Net cash provided by financing activities									535,806	370,742	267,285
Cash and Cash Equivalents, beginning of the year				127,927				102,170	127,927	102,170	118,616
Cash and Cash Equivalents, end of year	154,905				127,927				154,905	127,927	102,170
Parent
Cash Flow From Operating Activities:
Net income									96,070	81,051	65,384
Adjustments to Reconcile Net Income to Net Cash:
Share-based compensation									1,215	1,166	843
Distributions in excess of (equity in undistributed) income of subsidiaries									(71,164)	(53,551)	(31,087)
Gain on sale of insurance subsidiary											(8,265)
Gain on cancellation of subordinated debentures											(1,250)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments									50
Net Change in:
Other assets									3,358	833	(441)
Other liabilities									(1,900)	849	(1,486)
Investment in subsidiaries - operating activities									1,112	(1,938)	197
Net cash provided by operating activities									28,741	28,410	23,895
Cash Flows from Investing Activities:
Purchase of securities available for sale										(19,804)
Net cash received (paid) in acquisition									37		(14,500)
Proceeds from business divestitures											16,000
Other											575
Net cash used in investing activities									37	(19,804)	2,075
Cash Flows from Financing Activities:
Cash dividends									(31,820)	(22,203)	(15,654)
Repayment of borrowings											(3,750)
Stock issued under employee benefit plans									519	462	460
Stock issued under dividend reinvestment and stock purchase plan									991	835	661
Stock options exercised									2,398	2,602	1,531
Stock redeemed									(1,283)	(1,980)	(1,640)
Net cash provided by financing activities									(29,195)	(20,284)	(18,392)
Cash and Cash Equivalents, beginning of the year				$ 31,944				$ 43,622	31,944	43,622	36,044
Cash and Cash Equivalents, end of year	$ 31,527				$ 31,944				$ 31,527	$ 31,944	$ 43,622