Comprehensive Income - Schedule of Amounts Reclassed Out of Each Component of Accumulated Other Comprehensive Income (Detail) - Successor [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	$ (2,605)	$ (19,581)
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, after Tax	1,114
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(2,044)	8,497
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains	(4,563)	8,835
Net of tax and noncontrolling interest	(2,855)	8,835
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(2,855)	8,835
Accumulated Gain (Loss), Financial Liability, Fair Value Option, Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, after Tax	(1,114)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,114
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, before Tax, after Reclassification Adjustment	(1,344)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax and noncontrolling interest	(2,044)	8,497
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)	(1,708)	0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss), Financial Liability, Fair Value Option, Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	230
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	(303)	(338)
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	(241)	(241)
Actuarial gains (losses)	(62)	(97)
Total before tax	(303)	(338)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	$ (303)	$ (338)