Summary of Held-to-Maturity and Available-for-Sale Securities Having Continuous Unrealized Loss Position (Parenthetical) (Detail) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Unrealized Loss	$ 5,202,000	$ 8,506,000
Equity Securities
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Unrealized Loss	1,200,000
Equity Securities | US Large Cap Equity Securities
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Unrealized Loss	556,000
Principal balance of fund	29,100,000
Equity Securities | Financial Institution
Schedule of Investments [Line Items]
Temporarily Impaired Available-for-Sale Securities, Twelve Months or Longer Unrealized Loss	623,000
Principal balance of fund	$ 1,700,000