Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Fair Value Of Securities Deposited With Governmental Authorities	$ 6,281	$ 5,975
Fair Value of Securities Deposited in Connection wtih Letter of Credit	1,421
Market Value of Secured Note Collateral	205,882	209,983
Investment Portfolio [Member]
Investment [Line Items]
Fair Value of Cash and Securities Pledged to Derivative Counterparties	135,258	102,904
Ambac Assurance [Member]
Investment [Line Items]
Unsettled Amounts Secured Notes from LSNI	$ 12,561	$ 19,405