Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Phantom) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Ambac UK [Member]
Investment [Line Items]
Corporate and Asset Backed Securities Fair Value Disclosure	$ 169,943	$ 170,280