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Investments - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Sep. 30, 2017
Investment [Line Items]        
Securities fair value $ 5,926   $ 5,974  
Fair Value of Securities Placed in Trust 256,553   346,212 $ 256,553
Successor [Member]        
Investment [Line Items]        
Other investments 419,896   431,630  
Unrealized losses on securities 28,374   86,480  
Other investments (311) $ 8,409    
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 40,045   108,043  
Below Investment Grade Securities and Non-Rated Securities [Member] | Successor [Member]        
Investment [Line Items]        
Fair value of below investment grade securities and non-rated securities 694,796   1,855,694  
Gross unrealized loss 18,330   100,503  
Gains (losses) on securities held as of reporting date [Member] | Successor [Member]        
Investment [Line Items]        
Other investments (3,512) $ 5,928    
Ambac UK [Member]        
Investment [Line Items]        
Asset Backed Securities Fair Value Disclosure 169,943   170,280  
Interest Rate Contract [Member] | Successor [Member]        
Investment [Line Items]        
Other investments 142,856   136,603  
Reported Value Measurement [Member] | Interest Rate Contract [Member] | Successor [Member]        
Investment [Line Items]        
Other investments $ 2,853   $ 2,823