Loans - Investment in Loans by Portfolio Segment and Based on Impairment (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance				$ 21,116,000				$ 21,130,000	$ 21,116,000	$ 21,130,000	$ 42,271,000
Provision for loan losses	194,000	116,000	(1,614,000)	33,000	282,000	657,000	1,113,000	2,064,000	(1,271,000)	4,116,000	39,325,000
Charge-offs									(4,239,000)	(8,627,000)	(24,268,000)
Recoveries									2,081,000	4,497,000	1,960,000
Net (charge-offs)recovery from asset sale											(38,158,000)
Allowance, Ending balance	17,687,000				21,116,000				17,687,000	21,116,000	21,130,000
Loans individually evaluated for impairment	3,310,000				3,325,000				3,310,000	3,325,000
Loans collectively evaluated for impairment	14,377,000				17,791,000				14,377,000	17,791,000
Allowance, Ending balance	17,687,000				21,116,000				17,687,000	21,116,000	21,130,000
Loans individually evaluated for impairment	45,897,000				48,181,000				45,897,000	48,181,000
Loans collectively evaluated for impairment	1,118,986,000				1,000,984,000				1,118,986,000	1,000,984,000
Total loans	1,164,883,000				1,049,165,000				1,164,883,000	1,049,165,000
Commercial Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance				6,984,000				9,156,000	6,984,000	9,156,000	29,427,000
Provision for loan losses									(649,000)	(491,000)	20,306,000
Charge-offs									(1,656,000)	(5,208,000)	(19,049,000)
Recoveries									1,011,000	3,527,000	1,056,000
Net (charge-offs)recovery from asset sale											(22,584,000)
Allowance, Ending balance	5,690,000				6,984,000				5,690,000	6,984,000	9,156,000
Loans individually evaluated for impairment	717,000				791,000				717,000	791,000
Loans collectively evaluated for impairment	4,973,000				6,193,000				4,973,000	6,193,000
Allowance, Ending balance	5,690,000				6,984,000				5,690,000	6,984,000	9,156,000
Loans individually evaluated for impairment	14,845,000				14,154,000				14,845,000	14,154,000
Loans collectively evaluated for impairment	238,066,000				211,858,000				238,066,000	211,858,000
Total loans	252,911,000				226,012,000				252,911,000	226,012,000
Residential Mortgage Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance				9,830,000				7,515,000	9,830,000	7,515,000	8,268,000
Provision for loan losses									(550,000)	3,598,000	16,298,000
Charge-offs									(1,005,000)	(1,536,000)	(2,479,000)
Recoveries									242,000	253,000	180,000
Net (charge-offs)recovery from asset sale											(14,752,000)
Allowance, Ending balance	8,517,000				9,830,000				8,517,000	9,830,000	7,515,000
Loans individually evaluated for impairment	1,751,000				1,675,000				1,751,000	1,675,000
Loans collectively evaluated for impairment	6,766,000				8,155,000				6,766,000	8,155,000
Allowance, Ending balance	8,517,000				9,830,000				8,517,000	9,830,000	7,515,000
Loans individually evaluated for impairment	19,209,000				20,206,000				19,209,000	20,206,000
Loans collectively evaluated for impairment	712,009,000				613,716,000				712,009,000	613,716,000
Total loans	731,218,000				633,922,000				731,218,000	633,922,000
Consumer Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance				4,302,000				4,459,000	4,302,000	4,459,000	4,576,000
Provision for loan losses									(72,000)	1,009,000	2,721,000
Charge-offs									(1,578,000)	(1,883,000)	(2,740,000)
Recoveries									828,000	717,000	724,000
Net (charge-offs)recovery from asset sale											(822,000)
Allowance, Ending balance	3,480,000				4,302,000				3,480,000	4,302,000	4,459,000
Loans individually evaluated for impairment	842,000				859,000				842,000	859,000
Loans collectively evaluated for impairment	2,638,000				3,443,000				2,638,000	3,443,000
Allowance, Ending balance	3,480,000				4,302,000				3,480,000	4,302,000	4,459,000
Loans individually evaluated for impairment	11,843,000				13,821,000				11,843,000	13,821,000
Loans collectively evaluated for impairment	168,911,000				175,410,000				168,911,000	175,410,000
Total loans	$ 180,754,000				$ 189,231,000				$ 180,754,000	$ 189,231,000