Impaired Loans by Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Average impaired loans for the period	$ 44,738	$ 57,438
Contractual interest income due on impaired loans during the period	821	4,433
Interest income on impaired loans recognized on a cash basis	1,041	1,121
Interest income on impaired loans recognized on an accrual basis	141	560
Commercial
Financing Receivable, Impaired [Line Items]
Average impaired loans for the period	8,436	8,659
Contractual interest income due on impaired loans during the period	74	527
Interest income on impaired loans recognized on a cash basis	30	121
Interest income on impaired loans recognized on an accrual basis	53	257
All Other
Financing Receivable, Impaired [Line Items]
Average impaired loans for the period	6,486	6,499
Contractual interest income due on impaired loans during the period	118	112
Interest income on impaired loans recognized on a cash basis	15	82
Interest income on impaired loans recognized on an accrual basis	51	201
Commercial Real Estate Portfolio Segment | Commercial Acquisition, Development and Construction
Financing Receivable, Impaired [Line Items]
Average impaired loans for the period	6,079	12,270
Contractual interest income due on impaired loans during the period	152	1,222
Interest income on impaired loans recognized on a cash basis	50	323
Interest income on impaired loans recognized on an accrual basis	0	0
Commercial Real Estate Portfolio Segment | Commercial Investor Real Estate
Financing Receivable, Impaired [Line Items]
Average impaired loans for the period	12,524	13,838
Contractual interest income due on impaired loans during the period	269	1,181
Interest income on impaired loans recognized on a cash basis	169	175
Interest income on impaired loans recognized on an accrual basis	7	0
Commercial Real Estate Portfolio Segment | Commercial Owner Occupied Real Estate
Financing Receivable, Impaired [Line Items]
Average impaired loans for the period	11,214	16,172
Contractual interest income due on impaired loans during the period	208	1,391
Interest income on impaired loans recognized on a cash basis	777	420
Interest income on impaired loans recognized on an accrual basis	$ 30	$ 102