ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 7,127,199
Total other comprehensive income (loss)	(576,444)	$ 161,733
Ending Balance	6,392,342		$ 7,127,199
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	340,200		(6,300)	$ 424,100
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	12,696	(654,523)	(654,523)
Other comprehensive income (loss) before reclassifications	(577,225)		684,247
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	681		7,153
Amounts reclassified from accumulated other comprehensive income (loss)	100		1,594
Total other comprehensive income (loss)	(576,444)		692,994
Cumulative effect adjustments	(10,552)		(25,775)
Ending Balance	(574,300)		12,696	(654,523)
Unrealized Gains and Losses on Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	25,874	(656,322)	(656,322)
Other comprehensive income (loss) before reclassifications	(577,712)		700,536
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	681		7,153
Amounts reclassified from accumulated other comprehensive income (loss)	11		642
Total other comprehensive income (loss)	(577,020)		708,331
Cumulative effect adjustments	(10,552)		(26,135)
Ending Balance	(561,698)		25,874	(656,322)
Accumulated Gain and Loss Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	747	727	727
Other comprehensive income (loss) before reclassifications	487		(563)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	0		0
Amounts reclassified from accumulated other comprehensive income (loss)	89		451
Total other comprehensive income (loss)	576		(112)
Cumulative effect adjustments	0		132
Ending Balance	1,323		747	727
Minimum Pension Liability Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(13,925)	$ 1,072	1,072
Other comprehensive income (loss) before reclassifications	0		(15,726)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	0		0
Amounts reclassified from accumulated other comprehensive income (loss)	0		501
Total other comprehensive income (loss)	0		(15,225)
Cumulative effect adjustments	0		228
Ending Balance	$ (13,925)		$ (13,925)	$ 1,072