Loans and Allowance for Loan Losses - Summary of Consumer Loans by Payment Status (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Credit Quality [Line Items]
Total Loans	$ 10,967,860	$ 9,506,094
Non-Performing [Member]
Credit Quality [Line Items]
Non-Performing Loans	76,892	77,453
Direct Installment [Member]
Credit Quality [Line Items]
Total Loans	1,579,312	1,467,236
Direct Installment [Member] | Performing [Member]
Credit Quality [Line Items]
Performing Loans	1,495,276	1,377,418
Direct Installment [Member] | Non-Performing [Member]
Credit Quality [Line Items]
Non-Performing Loans	13,820	10,577
Residential Mortgages [Member]
Credit Quality [Line Items]
Total Loans	1,231,796	1,086,739
Residential Mortgages [Member] | Performing [Member]
Credit Quality [Line Items]
Performing Loans	747,276	664,214
Residential Mortgages [Member] | Non-Performing [Member]
Credit Quality [Line Items]
Non-Performing Loans	15,467	14,013
Indirect Installment [Member]
Credit Quality [Line Items]
Total Loans	805,836	655,587
Indirect Installment [Member] | Performing [Member]
Credit Quality [Line Items]
Performing Loans	801,862	648,499
Indirect Installment [Member] | Non-Performing [Member]
Credit Quality [Line Items]
Non-Performing Loans	1,339	1,202
Consumer Lines of Credit [Member] | Performing [Member]
Credit Quality [Line Items]
Performing Loans	913,224	832,071
Consumer Lines of Credit [Member] | Non-Performing [Member]
Credit Quality [Line Items]
Non-Performing Loans	1,783	597
Other [Member]
Credit Quality [Line Items]
Total Loans	54,261	45,183
Other [Member] | Performing [Member]
Credit Quality [Line Items]
Performing Loans	53,610	45,183
Other [Member] | Non-Performing [Member]
Credit Quality [Line Items]
Non-Performing Loans	651
Originated Loans [Member]
Credit Quality [Line Items]
Total Loans	9,250,628	8,154,765
Originated Loans [Member] | Direct Installment [Member]
Credit Quality [Line Items]
Total Loans	1,509,096	1,387,995
Originated Loans [Member] | Residential Mortgages [Member]
Credit Quality [Line Items]
Total Loans	762,743	678,227
Originated Loans [Member] | Indirect Installment [Member]
Credit Quality [Line Items]
Total Loans	803,201	649,701
Originated Loans [Member] | Consumer Lines of Credit [Member]
Credit Quality [Line Items]
Total Loans	915,007	832,668
Originated Loans [Member] | Other [Member]
Credit Quality [Line Items]
Total Loans	$ 54,261	$ 45,183