Special Purpose Entities, Including Variable Interest Entities - Summary of Fair Value of Fixed Income Securities, by Asset-Type, Held by Consolidated Variable Interest Entities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Variable Interest Entities [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	$ 3,310,649	$ 3,451,149
Investments:
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	167,394	133,052
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,349	38,195
Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member]
Investments:
Loans Unpaid Principal Balance	2,478,571	3,418,044
Long Term Debt Unpaid Principal Balance	3,576,046	$ 4,552,643
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	138,042
Investments:
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	$ 24,672