Loans and Allowance for Loan Losses - Allowance for loan losses by portfolio (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Ending balance of allowance attributable to loans:
Individually evaluated for impairment		$ 95,197	$ 104,385
Collectively evaluated for impairment		736,874	738,375
Total allowance for loan losses, covered		0	0	$ 0	$ 0
Total allowance for loan losses, total loans		832,071	842,760	834,106	838,293
Ending balance of loans:
Individually evaluated for impairment		517,083	569,294
Collectively evaluated for impairment		61,690,778	61,054,474
Total		62,207,861	61,623,768
Commercial, Financial and Agricultural
Ending balance of allowance attributable to loans:
Individually evaluated for impairment		52,896	61,705
Collectively evaluated for impairment		345,247	358,930
Total allowance for loan losses		398,143	420,635	459,214	458,580
Ending balance of loans:
Individually evaluated for impairment		256,162	307,680
Collectively evaluated for impairment		26,102,965	25,442,269
Total		26,359,127	25,749,949
Commercial Real Estate
Ending balance of allowance attributable to loans:
Individually evaluated for impairment	[1]	11,568	9,864
Collectively evaluated for impairment	[1]	110,207	108,269
Total allowance for loan losses	[1]	121,775	118,133	114,607	116,937
Ending balance of loans:
Individually evaluated for impairment	[1]	96,297	85,180
Collectively evaluated for impairment	[1]	13,588,846	13,912,517
Total	[1]	13,685,143	13,997,697
Residential Real Estate
Ending balance of allowance attributable to loans:
Individually evaluated for impairment	[2]	27,916	30,613
Collectively evaluated for impairment	[2]	77,938	79,243
Total allowance for loan losses	[2]	105,854	109,856	115,945	119,484
Ending balance of loans:
Individually evaluated for impairment	[2]	161,299	172,857
Collectively evaluated for impairment	[2]	16,148,085	16,209,259
Total	[2]	16,309,384	16,382,116
Consumer
Ending balance of allowance attributable to loans:
Individually evaluated for impairment	[3]	2,817	2,203
Collectively evaluated for impairment	[3]	203,482	191,933
Total allowance for loan losses	[3]	206,299	194,136	$ 144,340	$ 143,292
Ending balance of loans:
Individually evaluated for impairment	[3]	3,325	3,577
Collectively evaluated for impairment	[3]	5,850,882	5,490,429
Total	[3]	$ 5,854,207	$ 5,494,006

[1] Includes commercial real estate – mortgage and real estate – construction loans.

[2] Includes residential real estate – mortgage, equity lines of credit and equity loans.

[3] Includes credit card, consumer direct and consumer indirect loans.