Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Loan portfolio by risk-rating grades
Total	$ 9,122,907	$ 7,620,322
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	1,313,344	1,039,393
Real estate – construction
Loan portfolio by risk-rating grades
Total	736,985	633,389
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	2,762,683	2,343,721
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	4,112,585	3,427,530
Installment loans to individuals
Loan portfolio by risk-rating grades
Total	197,310	176,289
Purchased loans
Loan portfolio by risk-rating grades
Total	2,912,669	2,031,766
Purchased loans | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,958,864	1,284,431
Purchased loans | Pass
Loan portfolio by risk-rating grades
Total	1,855,624	1,241,834
Purchased loans | Watch
Loan portfolio by risk-rating grades
Total	84,956	24,595
Purchased loans | Substandard
Loan portfolio by risk-rating grades
Total	18,284	18,002
Purchased loans | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	495,545	275,570
Purchased loans | Commercial, financial, agricultural | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	451,224	248,993
Purchased loans | Commercial, financial, agricultural | Pass
Loan portfolio by risk-rating grades
Total	409,790	241,195
Purchased loans | Commercial, financial, agricultural | Watch
Loan portfolio by risk-rating grades
Total	38,947	4,974
Purchased loans | Commercial, financial, agricultural | Substandard
Loan portfolio by risk-rating grades
Total	2,487	2,824
Purchased loans | Real estate – construction
Loan portfolio by risk-rating grades
Total	112,093	85,731
Purchased loans | Real estate – construction | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	78,214	81,220
Purchased loans | Real estate – construction | Pass
Loan portfolio by risk-rating grades
Total	77,950	81,220
Purchased loans | Real estate – construction | Watch
Loan portfolio by risk-rating grades
Total	0	0
Purchased loans | Real estate – construction | Substandard
Loan portfolio by risk-rating grades
Total	264	0
Purchased loans | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	761,913	614,187
Purchased loans | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	147,720	100,039
Purchased loans | Real estate – 1-4 family mortgage | Pass
Loan portfolio by risk-rating grades
Total	134,136	91,369
Purchased loans | Real estate – 1-4 family mortgage | Watch
Loan portfolio by risk-rating grades
Total	7,591	2,498
Purchased loans | Real estate – 1-4 family mortgage | Substandard
Loan portfolio by risk-rating grades
Total	5,993	6,172
Purchased loans | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	1,503,075	1,037,454
Purchased loans | Real estate – commercial mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,279,984	853,498
Purchased loans | Real estate – commercial mortgage | Pass
Loan portfolio by risk-rating grades
Total	1,232,028	827,372
Purchased loans | Real estate – commercial mortgage | Watch
Loan portfolio by risk-rating grades
Total	38,418	17,123
Purchased loans | Real estate – commercial mortgage | Substandard
Loan portfolio by risk-rating grades
Total	9,538	9,003
Purchased loans | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	40,043	18,824
Purchased loans | Installment loans to individuals | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,722	681
Purchased loans | Installment loans to individuals | Pass
Loan portfolio by risk-rating grades
Total	1,720	678
Purchased loans | Installment loans to individuals | Watch
Loan portfolio by risk-rating grades
Total	0	0
Purchased loans | Installment loans to individuals | Substandard
Loan portfolio by risk-rating grades
Total	$ 2	$ 3