ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Thousands	1 Months Ended	11 Months Ended	12 Months Ended
	Jan. 31, 2015	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance		$ 5,554,059	$ 5,471,521	$ 4,581,224
Cumulative effect adjustments			0
Balance	$ 5,554,059	4,581,224	7,127,199	5,471,521
Unrealized gains and losses on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance		0	(656,322)	(1,247,065)
Other comprehensive income (loss) before reclassifications		(1,264,034)	700,536	606,985
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings		(393)	7,153	(6,782)
Amounts reclassified from accumulated other comprehensive income (loss)		17,362	642	(9,460)
Net current-period other comprehensive income (loss)		(1,247,065)	708,331	590,743
Cumulative effect adjustments			(26,135)
Balance	0	(1,247,065)	25,874	(656,322)
Offset of net unrealized losses in AOCI due to impact those net unrealized losses would have on certain of the Company's insurance assets and liabilities had the net unrealized losses been recognized in net income		623,000	(6,300)	424,100
Unrealized gains and losses on available-for-sale securities | Predecessor
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	1,484,169	1,962,130
Other comprehensive income (loss) before reclassifications	482,370
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	(243)
Amounts reclassified from accumulated other comprehensive income (loss)	(4,166)
Net current-period other comprehensive income (loss)	477,961
Balance	1,962,130				$ 1,484,169
Offset of net unrealized losses in AOCI due to impact those net unrealized losses would have on certain of the Company's insurance assets and liabilities had the net unrealized losses been recognized in net income	(492,600)				(504,400)
Accumulated Gain and Loss Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance		0	727	0
Other comprehensive income (loss) before reclassifications		(86)	(563)	688
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings		0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)		86	451	39
Net current-period other comprehensive income (loss)		0	(112)	727
Cumulative effect adjustments			132
Balance	0	0	747	727
Accumulated Gain and Loss Derivatives | Predecessor
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(82)	(50)
Other comprehensive income (loss) before reclassifications	9
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	0
Amounts reclassified from accumulated other comprehensive income (loss)	23
Net current-period other comprehensive income (loss)	32
Balance	(50)				(82)
Minimum Pension Benefits Liability Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance		0	1,072	5,931
Other comprehensive income (loss) before reclassifications		5,931	(15,726)	(5,659)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings		0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)		0	501	800
Net current-period other comprehensive income (loss)		5,931	(15,225)	(4,859)
Cumulative effect adjustments			228
Balance	0	5,931	(13,925)	1,072
Minimum Pension Benefits Liability Adjustment | Predecessor
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(66,011)	(78,036)
Other comprehensive income (loss) before reclassifications	(12,527)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	0
Amounts reclassified from accumulated other comprehensive income (loss)	502
Net current-period other comprehensive income (loss)	(12,025)
Balance	(78,036)				(66,011)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance		0	(654,523)	(1,241,134)
Other comprehensive income (loss) before reclassifications		(1,258,189)	684,247	602,014
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings		(393)	7,153	(6,782)
Amounts reclassified from accumulated other comprehensive income (loss)		17,448	1,594	(8,621)
Net current-period other comprehensive income (loss)		(1,241,134)	692,994	586,611
Cumulative effect adjustments			(25,775)
Balance	0	(1,241,134)	$ 12,696	$ (654,523)
AOCI Attributable to Parent | Predecessor
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	1,418,076	$ 1,884,044
Other comprehensive income (loss) before reclassifications	469,852
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	(243)
Amounts reclassified from accumulated other comprehensive income (loss)	(3,641)
Net current-period other comprehensive income (loss)	465,968
Balance	$ 1,884,044				$ 1,418,076