Investments - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investment [Line Items]
Other investments	$ 469,438	$ 391,217
Securities fair value	7,613	5,975
Ambac Assurance [Member]
Investment [Line Items]
Unsettled Amounts Secured Notes from LSNI	0	19,405
Ambac UK [Member]
Investment [Line Items]
Asset Backed Securities Fair Value Disclosure	0	144,672
Interest Rate Contract [Member]
Investment [Line Items]
Other investments	233,904	177,357
Reported Value Measurement [Member] | Interest Rate Contract [Member]
Investment [Line Items]
Other investments	78,998	$ 27,154
Commitments [Member] | Partnership Interest [Member]
Investment [Line Items]
Other investments	$ 25,000