Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017
Investment [Line Items]
Fair Value Of Securities Deposited With Governmental Authorities	$ 5,926	$ 5,974
Fair Value of Securities Placed in Trust	256,553	346,212	$ 256,553
Market Value of Secured Note Collateral	367,816
Successor [Member] | Cash and Securities Pledged Directly from Investment Portfolio [Member]
Investment [Line Items]
Fair Value of Cash and Securities Pledged to Derivative Counterparties	$ 107,060	$ 120,645