Investment Securities - Investment Securities Held to Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 454,648	$ 357,500
Gross unrealized gains	10,996	5,753
Gross unrealized losses	(2,367)	(7,340)
Fair value	463,277	355,913
Agency Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	7,426	7,523
Gross unrealized gains	26	13
Gross unrealized losses	(21)	(66)
Fair value	7,431	7,470
Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	3,693	5,273
Gross unrealized gains	173	247
Gross unrealized losses	0	0
Fair value	3,866	5,520
State and Municipal Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	433,464	334,750
Gross unrealized gains	10,737	5,435
Gross unrealized losses	(2,333)	(7,198)
Fair value	441,868	332,987
Corporate Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	10,065	9,954
Gross unrealized gains	60	58
Gross unrealized losses	(13)	(76)
Fair value	$ 10,112	$ 9,936