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Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Detail) (USD $)
In Thousands, unless otherwise specified
 9 Months Ended
Sep. 30, 2013
Dec. 31, 2012
Sep. 30, 2013
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Sep. 30, 2013
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Cumulative Probability of Earning Maximum Rate Until Maturity [Member]
Sep. 30, 2013
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Cumulative Probability of Principal Return Prior to Maturity [Member]
Sep. 30, 2013
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Cumulative Probability of Default [Member]
Sep. 30, 2013
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Liquidity Risk Premium [Member]
Sep. 30, 2013
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Recovery Rate in Default [Member]
Sep. 30, 2013
Minimum [Member]
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Cumulative Probability of Earning Maximum Rate Until Maturity [Member]
Sep. 30, 2013
Minimum [Member]
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Cumulative Probability of Principal Return Prior to Maturity [Member]
Sep. 30, 2013
Minimum [Member]
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Cumulative Probability of Default [Member]
Sep. 30, 2013
Minimum [Member]
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Liquidity Risk Premium [Member]
Sep. 30, 2013
Minimum [Member]
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Recovery Rate in Default [Member]
Sep. 30, 2013
Maximum [Member]
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Cumulative Probability of Earning Maximum Rate Until Maturity [Member]
Sep. 30, 2013
Maximum [Member]
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Cumulative Probability of Principal Return Prior to Maturity [Member]
Sep. 30, 2013
Maximum [Member]
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Cumulative Probability of Default [Member]
Sep. 30, 2013
Maximum [Member]
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Liquidity Risk Premium [Member]
Sep. 30, 2013
Maximum [Member]
Failed Auction Securities [Member]
Significant Unobservable Inputs (Level 3) [Member]
Recovery Rate in Default [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]                                    
Estimated Fair Value $ 6,231 $ 6,736 $ 4,982                              
Valuation Technique     Discounted cash flow                              
Unobservable Input       Cumulative probability of earning the maximum rate until maturity Cumulative probability of principal return prior to maturity Cumulative probability of default Liquidity risk premium Recovery rate in default                    
Weighted Average Interest Rate       0.03% 84.92% 15.05% 5.00% 40.00% 0.01% 74.24% 4.39% 5.00% 40.00% 0.05% 95.60% 25.71% 5.00% 40.00%