Summary of Insurance Enhanced Municipal Bonds Backed by Financial Guarantors (Detail) $ in Thousands	Mar. 31, 2019 USD ($)
Pre-Refunded Securities
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced municipal bonds	$ 0
Municipal Bond Insurance Association
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced municipal bonds	699
Municipal Bond Insurance Association | Pre-Refunded Securities
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced municipal bonds	0
Municipal Bond Insurance Association | Government Guaranteed Securities
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced municipal bonds	0
Municipal Bond Insurance Association | Exposure Net Of Pre-refunded & Government Guaranteed Securities
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced municipal bonds	699
Financial Guarantors
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced municipal bonds	699
Financial Guarantors | Pre-Refunded Securities
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced municipal bonds	0
Financial Guarantors | Government Guaranteed Securities
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced municipal bonds	0
Financial Guarantors | Exposure Net Of Pre-refunded & Government Guaranteed Securities
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced municipal bonds	$ 699