Comprehensive Income - Schedule of Amounts Reclassed Out of Each Component of Accumulated Other Comprehensive Income (Detail) - Successor [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	$ (44,464)	$ (30,709)	$ (17,364)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	13,453	(18,496)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains	14,805	(17,478)
Net of tax and noncontrolling interest	14,805	(17,478)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	14,805	(17,478)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax and noncontrolling interest	13,453	(18,496)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	0	0
Other Postretirement Benefits Plan [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(1,352)	(1,018)
Other Postretirement Benefits Plan [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	0	0
Prior service cost	(963)	(666)
Actuarial gains (losses)	(389)	(352)
Total before tax	(1,352)	(1,018)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	$ (1,352)	$ (1,018)