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Investments - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2016
Dec. 31, 2015
Schedule of Investments [Line Items]        
Commercial Mortgage Loan     $ 1,450,300,000 $ 1,509,100,000
Other-than-temporary impairment losses included in accumulated other comprehensive income   $ 0 0 0
Non-taxable exchanges   3,000,000 11,100,000 72,400,000
Commercial Mortgage Loans        
Schedule of Investments [Line Items]        
Impaired loans     0 0
Commitments to fund borrowers troubled debt restructuring     0 0
Residential mortgage-backed securities ("RMBS")        
Schedule of Investments [Line Items]        
Other-than-temporary impairment losses   $ 0 0 $ 0
Past Due 90 to 179 Days        
Schedule of Investments [Line Items]        
Commercial Mortgage Loan     $ 4,900,000  
Predecessor | Residential mortgage-backed securities ("RMBS")        
Schedule of Investments [Line Items]        
Other-than-temporary impairment losses $ 0