Components of other intangible assets subject to amortization (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 94,855	$ 94,855
Finite Lived Intangible Assets Accumulated Amortization	51,609	46,674
Finite-Lived Intangible Assets, Net	43,246	48,181
Core Deposits Member
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Core Deposits, Gross	77,885	77,885
Finite Lived Intangible Assets Accumulated Amortization	47,682	43,627
Finite-Lived Intangible Assets, Net	30,203	34,258
Customer Relationships Member
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Customer Relationships, Gross	16,835	16,835
Finite Lived Intangible Assets Accumulated Amortization	3,837	2,974
Finite-Lived Intangible Assets, Net	12,998	13,861
Other Intangible Assets Member
Finite-Lived Intangible Assets [Line Items]
Other Finite-Lived Intangible Assets, Gross	135	135
Finite Lived Intangible Assets Accumulated Amortization	90	73
Finite-Lived Intangible Assets, Net	$ 45	$ 62