Loans and Leases - Summary of Consumer Loans by Payment Status (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Credit Quality [Line Items]
Total Loans and Leases	$ 20,998,766	$ 14,896,943
Direct installment
Credit Quality [Line Items]
Total Loans and Leases	1,905,535	1,844,399
Residential mortgages
Credit Quality [Line Items]
Total Loans and Leases	2,702,691	1,844,574
Indirect installment
Credit Quality [Line Items]
Total Loans and Leases	1,448,433	1,196,313
Originated Loans and Leases
Credit Quality [Line Items]
Total Loans and Leases	15,345,490	12,547,987
Originated Loans and Leases | Direct installment
Credit Quality [Line Items]
Total Loans and Leases	1,755,713	1,765,257
Originated Loans and Leases | Residential mortgages
Credit Quality [Line Items]
Total Loans and Leases	2,036,226	1,446,776
Originated Loans and Leases | Indirect installment
Credit Quality [Line Items]
Total Loans and Leases	1,448,268	1,196,110
Acquired Loans
Credit Quality [Line Items]
Total Loans and Leases	5,653,276	2,348,956
Acquired Loans | Direct installment
Credit Quality [Line Items]
Total Loans and Leases	149,822	79,142
Acquired Loans | Residential mortgages
Credit Quality [Line Items]
Total Loans and Leases	666,465	397,798
Acquired Loans | Indirect installment
Credit Quality [Line Items]
Total Loans and Leases	165	203
Total consumer loans | Originated Loans and Leases
Credit Quality [Line Items]
Total Loans and Leases	6,391,677	5,507,770
Total consumer loans | Originated Loans and Leases | Performing
Credit Quality [Line Items]
Total Loans and Leases	6,352,285	5,474,286
Total consumer loans | Originated Loans and Leases | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	39,392	33,484
Total consumer loans | Originated Loans and Leases | Direct installment
Credit Quality [Line Items]
Total Loans and Leases	1,755,713	1,765,257
Total consumer loans | Originated Loans and Leases | Direct installment | Performing
Credit Quality [Line Items]
Total Loans and Leases	1,739,060	1,750,305
Total consumer loans | Originated Loans and Leases | Direct installment | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	16,653	14,952
Total consumer loans | Originated Loans and Leases | Residential mortgages
Credit Quality [Line Items]
Total Loans and Leases	2,036,226	1,446,776
Total consumer loans | Originated Loans and Leases | Residential mortgages | Performing
Credit Quality [Line Items]
Total Loans and Leases	2,019,816	1,433,409
Total consumer loans | Originated Loans and Leases | Residential mortgages | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	16,410	13,367
Total consumer loans | Originated Loans and Leases | Indirect installment
Credit Quality [Line Items]
Total Loans and Leases	1,448,268	1,196,110
Total consumer loans | Originated Loans and Leases | Indirect installment | Performing
Credit Quality [Line Items]
Total Loans and Leases	1,445,833	1,193,930
Total consumer loans | Originated Loans and Leases | Indirect installment | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	2,435	2,180
Total consumer loans | Originated Loans and Leases | Consumer lines of credit
Credit Quality [Line Items]
Total Loans and Leases	1,151,470	1,099,627
Total consumer loans | Originated Loans and Leases | Consumer lines of credit | Performing
Credit Quality [Line Items]
Total Loans and Leases	1,147,576	1,096,642
Total consumer loans | Originated Loans and Leases | Consumer lines of credit | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	3,894	2,985
Total consumer loans | Acquired Loans
Credit Quality [Line Items]
Total Loans and Leases	1,410,775	678,716
Total consumer loans | Acquired Loans | Performing
Credit Quality [Line Items]
Total Loans and Leases	1,408,765	678,204
Total consumer loans | Acquired Loans | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	2,010	512
Total consumer loans | Acquired Loans | Direct installment
Credit Quality [Line Items]
Total Loans and Leases	149,822	79,142
Total consumer loans | Acquired Loans | Direct installment | Performing
Credit Quality [Line Items]
Total Loans and Leases	149,751	79,142
Total consumer loans | Acquired Loans | Direct installment | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	71	0
Total consumer loans | Acquired Loans | Residential mortgages
Credit Quality [Line Items]
Total Loans and Leases	666,465	397,798
Total consumer loans | Acquired Loans | Residential mortgages | Performing
Credit Quality [Line Items]
Total Loans and Leases	666,465	397,798
Total consumer loans | Acquired Loans | Residential mortgages | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	0	0
Total consumer loans | Acquired Loans | Indirect installment
Credit Quality [Line Items]
Total Loans and Leases	165	203
Total consumer loans | Acquired Loans | Indirect installment | Performing
Credit Quality [Line Items]
Total Loans and Leases	165	203
Total consumer loans | Acquired Loans | Indirect installment | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	0	0
Total consumer loans | Acquired Loans | Consumer lines of credit
Credit Quality [Line Items]
Total Loans and Leases	594,323	201,573
Total consumer loans | Acquired Loans | Consumer lines of credit | Performing
Credit Quality [Line Items]
Total Loans and Leases	592,384	201,061
Total consumer loans | Acquired Loans | Consumer lines of credit | Non-Performing
Credit Quality [Line Items]
Total Loans and Leases	$ 1,939	$ 512