Fair Value Measurements - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Contract	Dec. 31, 2017 USD ($)	Dec. 31, 2016
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Additional basis point fee need to receive for issuing a CDS on obligation	0.05%
Additional basis points fee increase on change of reference obligation spread	0.15%
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.70%	2.50%
Minimum [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Basis point fee received for issuing a CDS on reference obligation base fee	0.20%
Credit spread of reference obligation of a base transaction	0.80%
Maximum [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Basis point fee received for issuing a CDS on reference obligation hypothetical fee	0.25%
Credit spread of reference obligation of revised transaction	1.00%
Successor [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other investments	$ 419,896	$ 431,630
Available-for-sale Securities	$ 4,277,297	$ 5,309,161
Successor [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using dealer quotes	8.00%		6.00%
PercentageofInvestedPortfiliowhereProjectedCashFlowsConsistSolelyofDeferredAmountsandInterest		13.00%
Successor [Member] | Residential Mortgage-Backed Securities [Member] | Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available-for-sale Securities	$ 5,309	$ 736,017
Successor [Member] | Corporate Obligations [Member] | Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available-for-sale Securities	0	0
Successor [Member] | Collateralized Debt Obligations [Member] | Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available-for-sale Securities	$ 0	$ 0
Successor [Member] | Minimum [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using internal valuation models	2.00%	15.00%
Successor [Member] | Maximum [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using external pricing services	90.00%		79.00%
Variable Interest Entity [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	3.30%	3.10%
Variable Interest Entity, Primary Beneficiary [Member] | Successor [Member] | Corporate Obligations [Member] | Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Available-for-sale Securities	$ 2,955,763	$ 2,914,145
Credit Derivatives [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Number Of Credit Derivative Contracts On Adversely Classified Credit Listing | Contract	0