Loans and Allowance for Loan Losses - Changes in the carrying value of acquired credit impaired loans (Details)			3 Months Ended
	Nov. 30, 2017 USD ($)	Jan. 03, 2017 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2018 USD ($)
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Percentage of loss adjustment based on most current collateral value					90
Maximum
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Threshold limit of loans for risk assessment by loan officers					$ 500,000
Minimum
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Threshold limit of loans for risk assessment by loan officers					$ 100,000
Acquired credit impaired loans
Changes in the carrying value of acquired loans at the acquisition date
Balance at the beginning of the period			$ 485,119,000	$ 618,803,000
Net reductions for payments, foreclosures, and accretion			(32,951,000)	(22,072,000)
Change in the allowance for loan losses on acquired loans			90,000	543,000
Balance at the end of the period			452,258,000	597,274,000
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Balance at beginning of period			116,754,000	133,096,000
Contractual interest income			(7,078,000)	(8,696,000)
Accretion on acquired loans			(5,120,000)	(3,801,000)
Reclass of nonaccretable difference due to improvement in expected cash flows			2,474,000	9,335,000
Other changes, net			(52,000)	(76,000)
Balance at end of period			106,978,000	129,858,000
Decline in accretable yield balance			9,800,000
Accretion income			12,200,000
Improved expected cash flows			2,500,000
Contractual principal and interest			583,518,000	760,973,000		$ 626,691,000
Acquired credit impaired loans | Southeastern Bank Financial
Changes in the carrying value of acquired loans at the acquisition date
Balance at the end of the period		$ 60,981,000
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Contractual principal and interest		$ 78,963,000
Acquired credit impaired loans | Park Sterling Corporation
Changes in the carrying value of acquired loans at the acquisition date
Balance at the end of the period	$ 78,967,000
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Contractual principal and interest	113,584,000
Acquired non-credit impaired loans accounted under FASB 310 20
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Contractual principal and interest			2,400,000,000	3,300,000,000
After fair value adjustment			2,400,000,000	3,300,000,000
Fair value adjustment			$ 30,200,000	$ 55,300,000
Acquired non-credit impaired loans accounted under FASB 310 20 | Park Sterling Corporation
Changes in the carrying amount of accretable difference for acquired impaired and non-impaired loans
Contractual principal and interest	2,100,000,000
After fair value adjustment	2,200,000,000
Fair value adjustment	$ 46,500,000