Loans and Allowance for Credit Losses - Schedule of Recorded Investment in Residential and Consumer Loans Based on Payment Activity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment	$ 1,408,491	$ 1,310,627	$ 997,808
Residential real estate	1,625,354	1,519,156	1,356,233
Consumer Heloc [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment	373,120	360,320
Consumer Heloc [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment, Performing	370,047	357,205
Consumer Heloc [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment, Nonperforming	3,073	3,115
Consumer Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment	897,208	846,969
Consumer Auto [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment, Performing	895,946	845,708
Consumer Auto [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment, Nonperforming	1,262	1,261
Consumer - Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment	138,163	103,338
Consumer - Other [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment, Performing	136,653	101,811
Consumer - Other [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment, Nonperforming	1,510	1,527
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Residential real estate	1,625,354	1,519,156
Residential [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment, Performing	1,610,932	1,505,188
Residential [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivable recorded investment, Nonperforming	$ 14,422	$ 13,968