Condensed Financial Information (parent company only) - Statement of Cash Flows (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
Cash Flow From Operating Activities:
Net income	$ 41,713	$ 41,113	$ 39,634	$ 36,679	$ 24,383	$ 24,358	$ 24,136	$ 23,193	$ 159,139	$ 96,070	$ 81,051
Adjustments to Reconcile Net Income to Net Cash:
Share-based compensation									3,592	2,827	2,600
Net Change in:
Net cash provided by operating activities									180,235	126,502	103,719
Cash Flows from Investing Activities:
Purchase of securities available for sale									(370,284)	(479,045)	(281,470)
Net cash received in acquisition										54,536
Other									804	(4,047)	(4,490)
Net cash used in investing activities									(575,576)	(635,330)	(448,704)
Cash Flows from Financing Activities:
Stock issued under employee benefit plans									707	519	462
Stock issued under dividend reinvestment and stock purchase plan									1,211	991	835
Stock options exercised									1,598	2,398	2,602
Stock redeemed									(1,902)	(1,283)	(1,980)
Net cash provided by financing activities									379,683	535,806	370,742
Cash and Cash Equivalents, beginning of the year				154,905				127,927	154,905	127,927	102,170
Cash and Cash Equivalents, end of year	139,247				154,905				139,247	154,905	127,927
Parent
Cash Flow From Operating Activities:
Net income									159,139	96,070	81,051
Adjustments to Reconcile Net Income to Net Cash:
Share-based compensation									1,256	1,215	1,166
Distributions in excess of (equity in undistributed) income of subsidiaries									(68,329)	(71,164)	(53,551)
Loss on sale of available for sale securities										50
Net Change in:
Other assets									584	3,358	833
Other liabilities									274	(1,900)	849
Investment in subsidiaries - operating activities									841	1,112	(1,938)
Net cash provided by operating activities									93,765	28,741	28,410
Cash Flows from Investing Activities:
Purchase of securities available for sale											(19,804)
Net cash received in acquisition										37
Other									2,189
Net cash used in investing activities									2,189	37	(19,804)
Cash Flows from Financing Activities:
Cash dividends									(41,660)	(31,820)	(22,203)
Stock issued under employee benefit plans									707	519	462
Stock issued under dividend reinvestment and stock purchase plan									1,211	991	835
Stock options exercised									1,598	2,398	2,602
Stock redeemed									(1,902)	(1,283)	(1,980)
Net cash provided by financing activities									(40,046)	(29,195)	(20,284)
Cash and Cash Equivalents, beginning of the year				$ 31,527				$ 31,944	31,527	31,944	43,622
Cash and Cash Equivalents, end of year	$ 87,435				$ 31,527				$ 87,435	$ 31,527	$ 31,944