Allowance for Credit Losses - Investment in Loans, Net of Unearned Income on Impairment Methodology (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	$ 29,602	$ 51,179
Collectively evaluated for impairment	11,055,136	9,466,195
Loans, net of unearned income	11,084,738	9,689,638
Purchased with deteriorated credit quality
Investment in loans, net of unearned income on impairment methodology
Loans, net of unearned income		172,264
Commercial
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	17,670	8,460
Collectively evaluated for impairment	2,630,392	1,329,974
Loans, net of unearned income	2,648,062	1,367,972
Commercial | Purchased with deteriorated credit quality
Investment in loans, net of unearned income on impairment methodology
Loans, net of unearned income		29,538
Real estate – construction:
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	0	12,416
Collectively evaluated for impairment	773,119	813,204
Loans, net of unearned income	773,119	826,483
Real estate – construction: | Purchased with deteriorated credit quality
Investment in loans, net of unearned income on impairment methodology
Loans, net of unearned income		863
Real estate – 1-4 family mortgage
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	4,718	20,262
Collectively evaluated for impairment	2,755,676	2,810,808
Loans, net of unearned income	2,760,394	2,866,613
Real estate – 1-4 family mortgage | Purchased with deteriorated credit quality
Investment in loans, net of unearned income on impairment methodology
Loans, net of unearned income		35,543
Real estate – commercial mortgage:
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	6,596	9,550
Collectively evaluated for impairment	4,570,413	4,131,582
Loans, net of unearned income	4,577,009	4,244,265
Real estate – commercial mortgage: | Purchased with deteriorated credit quality
Investment in loans, net of unearned income on impairment methodology
Loans, net of unearned income		103,133
Installment and Other
Investment in loans, net of unearned income on impairment methodology
Individually evaluated for impairment	618	491
Collectively evaluated for impairment	242,608	380,627
Loans, net of unearned income	$ 243,226	384,305
Installment and Other | Purchased with deteriorated credit quality
Investment in loans, net of unearned income on impairment methodology
Loans, net of unearned income		$ 3,187