Condensed Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Common Stock	Surplus	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Total
Balance at Dec. 31, 2016	$ 60,576	$ 711,307	$ 370,916	$ (8,211)	$ 1,134,588
Balance (in shares) at Dec. 31, 2016	24,230,392
Increase (Decrease) in Stockholders' Equity
Comprehensive income			85,133	4,423	89,556
Cash dividends declared on common stock per share			(28,956)		(28,956)
Common stock issued for Southeastern Bank Financial Corp. acquisition	$ 12,446	422,163			434,609
Common stock issued for Southeastern Bank Financial Corp. acquisition (in shares)	4,978,338
Employee stock purchases	$ 17	536			553
Employee stock purchases (in shares)	6,742
Stock options exercised	$ 84	1,050			1,134
Stock options exercised (in shares)	33,896
Restricted stock awards	$ 51	(51)
Restricted stock awards (in shares)	20,683
Stock issued pursuant to restricted stock units.	$ 95	(95)
Stock issued pursuant to restricted stock units (in shares)	37,802
Common stock repurchased	$ (101)	(3,525)			(3,626)
Common stock repurchased (in shares)	(40,484)
Share-based compensation expense		4,967			4,967
Balance at Sep. 30, 2017	$ 73,168	1,136,352	427,093	(3,788)	$ 1,632,825
Balance (in shares) at Sep. 30, 2017	29,267,369				29,267,369
Balance at Dec. 31, 2017	$ 91,899	1,807,601	419,847	(10,427)	$ 2,308,920
Balance (in shares) at Dec. 31, 2017	36,759,656				36,759,656
Increase (Decrease) in Stockholders' Equity
Comprehensive income			129,867	(31,135)	$ 98,732
Cash dividends declared on common stock per share			(37,506)		(37,506)
AOCI reclassification to retained earnings from adoption of ASU 2018-02 | ASU 2018- 02			2,947	(2,947)
Employee stock purchases	$ 22	687			709
Employee stock purchases (in shares)	8,783
Stock options exercised	$ 83	948			1,031
Stock options exercised (in shares)	33,424
Restricted stock awards	$ 12	(12)
Restricted stock awards (in shares)	4,586
Stock issued pursuant to restricted stock units.	$ 99	(99)
Stock issued pursuant to restricted stock units (in shares)	39,541
Common stock repurchased - buyback plan	$ (250)	(8,094)			(8,344)
Common stock repurchased - buyback plan (in shares)	(100,000)
Common stock repurchased	$ (57)	(1,959)			(2,016)
Common stock repurchased (in shares)	(22,752)
Share-based compensation expense		6,613			6,613
Balance at Sep. 30, 2018	$ 91,808	$ 1,805,685	$ 515,155	$ (44,509)	$ 2,368,139
Balance (in shares) at Sep. 30, 2018	36,723,238				36,723,238