Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Loan portfolio by risk-rating grades
Total	$ 7,767,657	$ 7,620,322
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	987,818	1,039,393
Real estate – construction
Loan portfolio by risk-rating grades
Total	712,818	633,389
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	2,433,099	2,343,721
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	3,461,174	3,427,530
Installment loans to individuals
Loan portfolio by risk-rating grades
Total	172,748	176,289
Purchased loans
Loan portfolio by risk-rating grades
Total	1,709,891	2,031,766
Purchased loans | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	1,061,613	1,284,431
Purchased loans | Pass
Loan portfolio by risk-rating grades
Total	1,021,193	1,241,834
Purchased loans | Watch
Loan portfolio by risk-rating grades
Total	22,532	24,595
Purchased loans | Substandard
Loan portfolio by risk-rating grades
Total	17,888	18,002
Purchased loans | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Total	197,455	275,570
Purchased loans | Commercial, financial, agricultural | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	176,577	248,993
Purchased loans | Commercial, financial, agricultural | Pass
Loan portfolio by risk-rating grades
Total	168,753	241,195
Purchased loans | Commercial, financial, agricultural | Watch
Loan portfolio by risk-rating grades
Total	4,562	4,974
Purchased loans | Commercial, financial, agricultural | Substandard
Loan portfolio by risk-rating grades
Total	3,262	2,824
Purchased loans | Real estate – construction
Loan portfolio by risk-rating grades
Total	70,438	85,731
Purchased loans | Real estate – construction | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	69,410	81,220
Purchased loans | Real estate – construction | Pass
Loan portfolio by risk-rating grades
Total	67,609	81,220
Purchased loans | Real estate – construction | Watch
Loan portfolio by risk-rating grades
Total	1,538	0
Purchased loans | Real estate – construction | Substandard
Loan portfolio by risk-rating grades
Total	263	0
Purchased loans | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Total	520,649	614,187
Purchased loans | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	81,823	100,039
Purchased loans | Real estate – 1-4 family mortgage | Pass
Loan portfolio by risk-rating grades
Total	75,205	91,369
Purchased loans | Real estate – 1-4 family mortgage | Watch
Loan portfolio by risk-rating grades
Total	1,798	2,498
Purchased loans | Real estate – 1-4 family mortgage | Substandard
Loan portfolio by risk-rating grades
Total	4,820	6,172
Purchased loans | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Total	906,219	1,037,454
Purchased loans | Real estate – commercial mortgage | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	733,174	853,498
Purchased loans | Real estate – commercial mortgage | Pass
Loan portfolio by risk-rating grades
Total	708,999	827,372
Purchased loans | Real estate – commercial mortgage | Watch
Loan portfolio by risk-rating grades
Total	14,634	17,123
Purchased loans | Real estate – commercial mortgage | Substandard
Loan portfolio by risk-rating grades
Total	9,541	9,003
Purchased loans | Installment loans to individuals
Loan portfolio by risk-rating grades
Total	15,130	18,824
Purchased loans | Installment loans to individuals | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Total	629	681
Purchased loans | Installment loans to individuals | Pass
Loan portfolio by risk-rating grades
Total	627	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