Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Phantom) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Ambac UK [Member]
Investment [Line Items]
Corporate and Asset Backed Securities Fair Value Disclosure	$ 171,034	$ 118,813