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, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 5,471,521
Total other comprehensive income	161,733	$ 437,320
Ending Balance	5,564,820		$ 5,471,521
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	348,400		424,100	$ 623,000
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(654,523)	(1,241,134)	(1,241,134)
Other comprehensive income (loss) before reclassifications	158,969		602,014
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	3,703		(6,782)
Amounts reclassified from accumulated other comprehensive income (loss)	(939)		(8,621)
Total other comprehensive income	161,733		586,611
Ending Balance	(492,790)		(654,523)	(1,241,134)
Unrealized Gains and Losses on Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(656,322)	(1,247,065)	(1,247,065)
Other comprehensive income (loss) before reclassifications	159,641		606,985
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings	3,703		(6,782)
Amounts reclassified from accumulated other comprehensive income (loss)	(1,072)		(9,460)
Total other comprehensive income	162,272		590,743
Ending Balance	(494,050)		(656,322)	(1,247,065)
Accumulated Gain and Loss Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	727	0	0
Other comprehensive income (loss) before reclassifications	(672)		688
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)	133		39
Total other comprehensive income	(539)		727
Ending Balance	188		727	0
Minimum Pension Liability Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	1,072	$ 5,931	5,931
Other comprehensive income (loss) before reclassifications	0		(5,659)
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		800
Total other comprehensive income	0		(4,859)
Ending Balance	$ 1,072		$ 1,072	$ 5,931