Short-Term Borrowings - Composition of Short-Term Borrowings, Interest Expense and Weighted Average Interest Rate (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Balance	$ 320,605	$ 173,847	$ 137,582
Interest Expense	514	341	205
Weighted Average Interest Rate	0.27%	0.24%	0.19%
REPOs [Member]
Short-term Debt [Line Items]
Balance	30,605	33,847	62,582
Interest Expense	3	62	82
Weighted Average Interest Rate	0.01%	0.01%	0.20%
FHLB Advances [Member]
Short-term Debt [Line Items]
Balance	290,000	140,000	75,000
Interest Expense	$ 511	$ 279	$ 123
Weighted Average Interest Rate	0.31%	0.30%	0.19%