Loans and Allowance for Credit Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans and leases outstanding	$ 13,615,701	$ 12,117,524	$ 12,117,524
Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	71,660
Current	13,544,041
Total loans and leases outstanding	13,615,701	12,122,002
Commercial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans and leases outstanding	4,307,505	2,815,154	2,890,296
Commercial | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	7,563
Current	4,106,081
Total loans and leases outstanding	4,113,644	2,817,833
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans and leases outstanding	5,403,316	4,889,253	5,166,792
Commercial real estate | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	25,587
Current	5,211,360
Total loans and leases outstanding	5,236,947	4,890,890
BBCC
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans and leases outstanding		352,681
BBCC | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,379
Current	358,851
Total loans and leases outstanding	360,230	352,714
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans and leases outstanding	2,229,298	2,334,289	$ 2,334,289
Residential real estate | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	27,346
Current	2,201,952
Total loans and leases outstanding	2,229,298	2,334,394
Indirect
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans and leases outstanding	933,723	935,584
Indirect | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	4,558
Current	929,165
Total loans and leases outstanding	933,723	935,594
Direct
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans and leases outstanding	194,573	228,524
Direct | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,551
Current	193,022
Total loans and leases outstanding	194,573	228,526
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total loans and leases outstanding	547,286	562,039
Home equity | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	3,676
Current	543,610
Total loans and leases outstanding	547,286	$ 562,051
30-59 Days Past Due | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	20,977
30-59 Days Past Due | Commercial | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,229
30-59 Days Past Due | Commercial real estate | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	312
30-59 Days Past Due | BBCC | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	828
30-59 Days Past Due | Residential real estate | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	13,331
30-59 Days Past Due | Indirect | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	3,187
30-59 Days Past Due | Direct | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	856
30-59 Days Past Due | Home equity | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	1,234
60-89 Days Past Due | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	17,832
60-89 Days Past Due | Commercial | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	845
60-89 Days Past Due | Commercial real estate | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	10,728
60-89 Days Past Due | BBCC | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	416
60-89 Days Past Due | Residential real estate | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	4,319
60-89 Days Past Due | Indirect | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	580
60-89 Days Past Due | Direct | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	260
60-89 Days Past Due | Home equity | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	684
Past Due 90 Days or More | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	32,851
Past Due 90 Days or More | Commercial | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	5,489
Past Due 90 Days or More | Commercial real estate | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	14,547
Past Due 90 Days or More | BBCC | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	135
Past Due 90 Days or More | Residential real estate | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	9,696
Past Due 90 Days or More | Indirect | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	791
Past Due 90 Days or More | Direct | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	435
Past Due 90 Days or More | Home equity | Adjusted Balance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due	$ 1,758