Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Balance Sheet (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	May 01, 2013	Apr. 30, 2013
Condensed Balance Sheet Statements, Captions [Line Items]
Tolling Payment Payable to AFG	$ 30,496	$ 28,691
Preferred stock, par value				$ 0.01
Preferred stock, shares authorized				20,000,000
Common stock, par value				$ 0.01
Common stock, shares authorized				130,000,000
Common stock, shares issued				45,000,000
Successor [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Fixed income securities, amortized cost	$ 4,614,623	$ 5,435,385
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Preferred Stock, Shares Outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01	$ 0.01
Common stock, shares authorized	130,000,000	130,000,000
Common stock, shares issued			45,000,000
Parent Company [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Fixed income securities, amortized cost	$ 239,476	$ 220,749
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	130,000,000	130,000,000
Parent Company [Member] | Successor [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, shares issued	45,275,982	45,194,954
Common stock, shares outstanding	45,275,982	45,194,954
Treasury stock, shares	24,816	22,458