Investments in Tax Credit Entities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Net Investment Income [Line Items]
Generated Tax credits	$ 8,639,000	$ 4,808,000	$ 2,868,000
Tax credit as Percentage of project cost	39.00%
Credit allowance period	7 years
Qualified Equity Investments	118,000,000
CDE gross amount	118,000,000
Total assets	3,286,617,000	2,670,867,000
Cash	28,140,000	26,471,000
Loans	2,325,634,000	1,895,240,000
Other assets	23,781,000	20,915,000
Liabilities	2,904,758,000	2,422,765,000
Due Between One Year And Three Years [Member]
Net Investment Income [Line Items]
Investor receives a credit equal to the total amount invested in the project	5.00%
Due Between Year Four And Year Seven [Member]
Net Investment Income [Line Items]
Investor receives a credit equal to the total amount invested in the project	6.00%
Maximum [Member]
Net Investment Income [Line Items]
Received allocation amount	46,000,000
Federal NMTC [Member]
Net Investment Income [Line Items]
Received allocation amount	50,000,000	40,000,000	28,000,000
Generated Tax credits	19,500,000	15,600,000	10,900,000
Investments in QEI projects	The CDE is required to invest the proceeds of each QEI in projects located in or benefitting low-income communities, which are generally defined as those census tracts with poverty rates greater than 20% and/or median family incomes that are less than or equal to 80% of the area median family income.
CDE investments in QEI projects located in low-income communities with poverty rates	20.00%
CDE investments in QEI projects located in low-income communities with median family income	80.00%
Amount received as cumulative awards	118,000,000
Credit allowance period	6 years
Investments generated recognized	46,000,000
Investments generated recognized	10,400,000
Accrued Tax Credit	35,600,000
First National Banking Company [Member]
Net Investment Income [Line Items]
CDE gross amount	22,500,000
Lender [Member]
Net Investment Income [Line Items]
CDE gross amount	62,900,000
State [Member]
Net Investment Income [Line Items]
CDE gross amount	32,600,000
Certified Development Entities [Member]
Net Investment Income [Line Items]
Generated Tax credits	61,200,000
Total assets	68,700,000
Cash	27,900,000
Loans	40,700,000
Other assets	100,000
Liabilities	100,000
Capital	68,600,000
Investments	61,200,000
Loans related to real estate	81,800,000	64,100,000
Certified Development Entities [Member] | Investment In Limited Partnership [Member]
Net Investment Income [Line Items]
Generated Tax credits	60,600,000
Accrued Tax Credit	33,400,000
Tax credits recognized	27,200,000
Low-Income Housing tax credits [Member]
Net Investment Income [Line Items]
Accrued Tax Credit	47,400,000
Investments	40,000,000	25,300,000
Tax credits recognized	9,500,000
Investment in partnership expected to generate Low-Income Housing tax credit	57,000,000
Interest in property	99.90%
Property investment term	15 years
Loans related to real estate projects	42,000,000	42,000,000
Federal Historic tax credits [Member]
Net Investment Income [Line Items]
Generated Tax credits	25,700,000
Investments	16,500,000	7,100,000
Tax credits recognized	17,800,000
Interest in property	99.90%
Property investment term	10 years
Tax credits expected to be recognized during the year	$ 7,500,000
Community Development Fund [Member]
Net Investment Income [Line Items]
Qualified Equity Investment low Income community investment percentage	85.00%