Loans and Allowance for Loan Losses - Table of loan payment estimates - (Details) - USD ($) $ in Thousands	Nov. 30, 2017	Jan. 03, 2017	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Acquired credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Contractual principal and interest			$ 670,008		$ 795,850	$ 741,268
Non-accretable difference			(38,422)		(39,324)	(26,160)
Cash flows expected to be collected			631,586		756,526	715,108
Accretable Yield			(114,985)		(133,096)	(132,575)
Carrying value			516,601		623,430	582,533
Allowance for loan losses on acquired loans			(3,968)	$ (4,426)	(4,627)	(3,670)	$ (3,741)	$ (3,395)
Acquired credit impaired loans | Park Sterling Corporation
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Contractual principal and interest	$ 113,584
Non-accretable difference	(27,248)
Cash flows expected to be collected	86,336
Accretable difference	(7,369)
Carrying value	78,967
Acquired credit impaired loans | Southeastern Bank Financial
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Contractual principal and interest		$ 78,963
Non-accretable difference		(13,072)
Cash flows expected to be collected		65,891
Accretable difference		(4,910)
Carrying value		60,981
Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			2,786,102		3,507,907	1,455,555
Acquired non-credit impaired loans accounted under FASB 310 20 | Park Sterling Corporation
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Contractual principal and interest	2,100,000
After fair value adjustment	2,200,000
Fair value adjustment	$ 46,500
Acquired credit impaired loans accounted under FASB 310 30
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			516,601		623,430	582,533
Allowance for loan losses on acquired loans			(3,968)		(4,627)	(3,670)
Impaired non-acquired and acquired loans accounted under FASB ASC topic 310-20 | Southeastern Bank Financial
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Contractual principal and interest		1,000,000
Carrying value		986,500
Fair value adjustment		$ 18,800
Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			2,786,102		3,507,907	1,455,555
Residential real estate | Acquired credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			218,019		260,787	249,666
Allowance for loan losses on acquired loans			(2,311)	(2,514)	(3,553)	(2,776)	(2,741)	(2,419)
Residential real estate | Acquired credit impaired loans accounted under FASB 310 30
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			218,019		260,787	249,666
Commercial non-owner occupied real estate | Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			907,355		1,220,523	276,590
Commercial owner occupied real estate loan | Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			455,803		521,818	207,572
Commercial owner occupied real estate loan | Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			455,803		521,818	207,572
Consumer loans | Acquired credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			44,081			53,302
Allowance for loan losses on acquired loans			(546)	(572)	(461)	(462)	(548)	(558)
Consumer loans | Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			906,622		1,031,202	656,906
Consumer loans | Acquired credit impaired loans accounted under FASB 310 30
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			44,081		51,453	53,302
Consumer loans | Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			1,024,651		1,170,201	793,042
Commercial and industrial | Acquired credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			10,671		26,946	25,796
Allowance for loan losses on acquired loans			(79)	(128)	(145)	(243)	(320)	(238)
Commercial and industrial | Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			247,922		398,696	101,427
Commercial and industrial | Acquired credit impaired loans accounted under FASB 310 30
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			10,671		26,946	25,796
Commercial and industrial | Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			247,922		398,696	101,427
Other income producing property | Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			150,371		196,669	76,924
Other income producing property | Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			150,371		196,669	76,924
Consumer | Acquired credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			44,081		51,453	53,302
Consumer | Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			118,029		137,710	136,136
Commercial loans | Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			1,761,451		2,337,706	662,513
Other loans | Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value					1,289
Other loans | Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value					1,289
Construction and land development | Commercial non-owner occupied real estate | Acquired credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			34,312		49,649	46,248
Allowance for loan losses on acquired loans			(340)	(576)	(180)	(189)	(92)	(139)
Construction and land development | Commercial non-owner occupied real estate | Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			222,562		403,357	76,886
Construction and land development | Commercial non-owner occupied real estate | Acquired credit impaired loans accounted under FASB 310 30
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			34,312		49,649	46,248
Construction and land development | Commercial non-owner occupied real estate | Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			222,562		403,357	76,886
Other commercial non-owner occupied real estate | Commercial non-owner occupied real estate | Acquired credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			209,518		234,595	207,521
Allowance for loan losses on acquired loans			(692)	$ (636)	(288)		$ (40)	$ (41)
Other commercial non-owner occupied real estate | Commercial non-owner occupied real estate | Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			684,793		817,166	199,704
Other commercial non-owner occupied real estate | Commercial non-owner occupied real estate | Acquired credit impaired loans accounted under FASB 310 30
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			209,518		234,595	207,521
Other commercial non-owner occupied real estate | Commercial non-owner occupied real estate | Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			684,793		817,166	199,704
Home equity loans | Consumer loans | Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			259,558		320,591	164,291
Home equity loans | Consumer loans | Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			259,558		320,591	164,291
Consumer | Consumer loans | Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			118,029		137,710	136,136
Consumer Owner Occupied Loans | Consumer loans | Acquired non-credit impaired loans accounted under FASB 310 20
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			647,064		710,611	492,615
Consumer Owner Occupied Loans | Consumer loans | Acquired non-credit impaired loans
Contractual loan payments receivable, estimates of amounts not expected to be collected, other fair value adjustments and the resulting fair values of acquired loans
Carrying value			$ 647,064		$ 710,611	$ 492,615