Summary of Insurance Enhanced Municipal Bonds Backed by Financial Guarantors (Detail) $ in Thousands	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	$ 9,810
Ambac Financial Group
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	1,494
Municipal Bond Insurance Association
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	2,657
Gov't National Housing Association
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	495
Financial Guarantors
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	4,646
Other Credit Enhanced Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	5,164
Pre-Refunded Securities
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	5,614
Pre-Refunded Securities | Ambac Financial Group
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	450
Pre-Refunded Securities | Financial Guarantors
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	450
Pre-Refunded Securities | Other Credit Enhanced Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	5,164
Government Guaranteed Securities
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	495
Government Guaranteed Securities | Gov't National Housing Association
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	495
Government Guaranteed Securities | Financial Guarantors
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	495
Exposure Net Of Pre-refunded & Government Guaranteed Securities
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	3,701
Exposure Net Of Pre-refunded & Government Guaranteed Securities | Ambac Financial Group
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	1,044
Exposure Net Of Pre-refunded & Government Guaranteed Securities | Municipal Bond Insurance Association
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	2,657
Exposure Net Of Pre-refunded & Government Guaranteed Securities | Financial Guarantors
Schedule of Available-for-sale Securities [Line Items]
Investments in insurance enhanced municipal bonds	$ 3,701