Fair Value Measurements - Summary of Information about Described Model Inputs Used to Determine Fair Value of Each Class of Credit Derivatives (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Jun. 30, 2019
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other investments	$ 462,427	$ 391,217
Fair value of derivative liabilities	102,076
Real Estate [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other investments	15,321	16,123
Interest Rate Contract [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other investments	234,758	177,357
Illiquid Investments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other investments	$ 53,445	$ 84,297
Asset-backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Inputs Coupon Rate		5.97%	5.98%
Fair Value Inputs Maturity	15 years 9 months 3 days	16 years 3 months 14 days
Fair Value Inputs Yield		12.00%	10.50%
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value of derivative liabilities	$ 0
Variable Interest Entity, Primary Beneficiary [Member] | European Transactions [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Inputs Coupon Rate		2.20%
Fair Value Inputs Maturity		18 years 11 months 4 days
Fair Value Inputs Yield		3.18%