Fair Value Measurements - Summary of Information about Described Model Inputs Used to Determine Fair Value of Each Class of Credit Derivatives (Detail) - Successor [Member] $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Transactions $ / shares	Dec. 31, 2017 USD ($) Transactions $ / shares
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Number of Instruments Held | Transactions	2	2
Other investments	$ 419,896	$ 431,630
Other [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Notional outstanding	$ 330,656	$ 325,890
Weighted average reference obligation price | $ / shares	$ 98.6	$ 99.3
Weighted average life (WAL) in years	6 years 3 months	6 years 6 months
Weighted average credit rating	A	A
Weighted average relative change ratio	23.40%	23.60%
CVA percentage	9.25%	9.64%
Fair value of derivative liabilities	$ 1,018	$ 566
Real Estate [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	34,752	33,154
Hedge Funds, Multi-strategy [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	56,394	53,054
Interest Rate Contract [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	142,856	136,603
Illiquid Investments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	$ 71,366	$ 67,787
Asset-backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs Coupon Rate	5.97%	5.97%
Fair Value Inputs Maturity	16 years 10 months 2 days	17 years 9 days
Fair Value Inputs Yield	0.00%	12.00%
Variable Interest Entity, Primary Beneficiary [Member] | European Transactions [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs Coupon Rate	0.37%	0.40%
Fair Value Inputs Maturity	14 years 12 months 23 days	15 years 3 months 11 days
Fair Value Inputs Yield	5.35%	4.82%