Comprehensive Income - Schedule of Amounts Reclassed Out of Each Component of Accumulated Other Comprehensive Income (Detail) - Successor [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	$ (1,995)	$ (6,882)	$ (4,600)	$ (26,463)
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, after Tax	906		(208)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(47,342)	(2,759)	(51,094)	5,738
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains	(47,256)	(2,421)	(50,111)	6,414
Net of tax and noncontrolling interest	(46,134)	(2,421)	(50,697)	6,414
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(46,134)	(2,421)	(50,697)	6,414
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	(906)	0	208	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(906)		208
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, before Tax, after Reclassification Adjustment	(1,094)	0	250	0
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax and noncontrolling interest	(47,342)	(2,759)	(51,094)	5,738
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)	(1,122)	0	586	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)	(188)	0	42	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	(302)	(338)	(605)	(676)
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	0	0
Prior service cost	(241)	(241)	(482)	(482)
Actuarial gains (losses)	(61)	(97)	(123)	(194)
Total before tax	(302)	(338)	(605)	(676)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	$ (302)	$ (338)	$ (605)	$ (676)