Condensed Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Corporate securities	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			50,087	4,514	54,601
Cash dividends declared on common stock per share			(19,297)		(19,297)
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	$ 8	259			267
Employee stock purchases (in shares)	3,226
Stock options exercised	$ 84	1,050			1,134
Stock options exercised (in shares)	33,896
Restricted stock awards	$ 39	(39)
Restricted stock awards (in shares)	15,851
Stock issued pursuant to restricted stock units.	$ 95	(95)
Stock issued pursuant to restricted stock units (in shares)	37,802
Common stock repurchased	$ (100)	(3,505)			(3,605)
Common stock repurchased (in shares)	(40,241)
Share-based compensation expense		3,188			3,188
Balance at Jun. 30, 2017	$ 73,148	1,134,328	401,706	(3,697)	$ 1,605,485
Balance (in shares) at Jun. 30, 2017	29,259,264				29,259,264
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			82,785	(22,697)	$ 60,088
Cash dividends declared on common stock per share			(24,651)		(24,651)
AOCI reclassification to retained earnings from adoption of ASU 2018-02 | ASU 2018- 02			2,947	(2,947)
Employee stock purchases	$ 11	341			352
Employee stock purchases (in shares)	4,338
Stock options exercised	$ 83	948			1,031
Stock options exercised (in shares)	33,424
Restricted stock awards	$ 20	(20)
Restricted stock awards (in shares)	7,836
Stock issued pursuant to restricted stock units.	$ 99	(99)
Stock issued pursuant to restricted stock units (in shares)	39,541
Common stock repurchased	$ (48)	(1,678)			(1,726)
Common stock repurchased (in shares)	(19,239)
Share-based compensation expense		4,353			4,353
Balance at Jun. 30, 2018	$ 92,064	$ 1,811,446	$ 480,928	$ (36,071)	$ 2,348,367
Balance (in shares) at Jun. 30, 2018	36,825,556				36,825,556