Loans and Allowance for Credit Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	$ 78,400
Current	12,306,212
Total loans and leases outstanding	12,384,612	$ 12,117,524	$ 12,117,524
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	14,441
Current	2,830,024
Total loans and leases outstanding	2,844,465	2,815,154	2,890,296
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	21,726
Current	5,096,713
Total loans and leases outstanding	5,118,439	4,889,253	5,166,792
BBCC
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	809
Current	366,330
Total loans and leases outstanding	367,139	352,681
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	27,753
Current	2,300,098
Total loans and leases outstanding	2,327,851	2,334,289	2,334,289
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans and leases outstanding			$ 1,726,147
Indirect
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	7,660
Current	945,476
Total loans and leases outstanding	953,136	935,584
Direct
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	2,021
Current	209,772
Total loans and leases outstanding	211,793	228,524
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	3,990
Current	557,799
Total loans and leases outstanding	561,789	$ 562,039
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	33,632
30-59 Days Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	3,578
30-59 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	6,114
30-59 Days Past Due | BBCC
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	419
30-59 Days Past Due | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	13,895
30-59 Days Past Due | Indirect
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	6,358
30-59 Days Past Due | Direct
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,513
30-59 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,755
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	10,286
60-89 Days Past Due | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,707
60-89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	2,200
60-89 Days Past Due | BBCC
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	156
60-89 Days Past Due | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	4,603
60-89 Days Past Due | Indirect
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	963
60-89 Days Past Due | Direct
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	183
60-89 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	474
Past Due 90 Days or More
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	34,482
Past Due 90 Days or More | Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	9,156
Past Due 90 Days or More | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	13,412
Past Due 90 Days or More | BBCC
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	234
Past Due 90 Days or More | Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	9,255
Past Due 90 Days or More | Indirect
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	339
Past Due 90 Days or More | Direct
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	325
Past Due 90 Days or More | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	$ 1,761