Securities - Components of Available for Sale Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 583,020	$ 566,509
Available For Sale Securities, Gross Unrealized Gain Accumulated In Investments	386	8,763
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	(28,218)	(710)
Total fair value	555,188	574,562
U.S. Treasury and government sponsored entities' securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	250,538	161,845
Available For Sale Securities, Gross Unrealized Gain Accumulated In Investments		2,409
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	(16,671)	(562)
Total fair value	233,867	163,692
Mortgage-backed GSE securities: residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	101	101
Available For Sale Securities, Gross Unrealized Gain Accumulated In Investments	269	212
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments
Total fair value	370	313
Mortgage-backed GSE securities: residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	332,381	404,563
Available For Sale Securities, Gross Unrealized Gain Accumulated In Investments	117	6,142
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	(11,547)	(148)
Total fair value	$ 320,951	$ 410,557