Investments - Summary of Net Investment Income (Detail) - Successor [Member] - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities									$ 18,242	$ 27,654	$ 12,615
Trading Securities, Realized Gain (Loss)									4,854	7,474	4,966
Fixed income securities									337,454	288,554	257,404
Short-term investments									7,898	1,505	299
Loans									520	337	420
Investment expense									(8,098)	(9,347)	(8,786)
Securities available-for-sale and short-term									337,774	281,049	249,337
Other investments									23,179	32,318	16,952
Total net investment income	$ 107,057	$ 87,177	$ 85,160	$ 81,559	$ 90,871	$ 90,917	$ 70,758	$ 60,821	360,953	313,367	266,289
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Other investments									$ 13,388	$ 20,180	$ 7,649