ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 0
Other comprehensive income (loss) before reclassifications	(28,027)	$ 32,945
Amounts reclassified from accumulated other comprehensive income	12,497	(9,415)	$ (7,586)
Other comprehensive income (loss)	(15,530)	23,530	(12,352)
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA		0
Unrealized gains (losses) on available-for- sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	83,497	49,748
Cumulative Effect of New Accounting Principle in Period of Adoption	(66,234)
Other comprehensive income (loss) before reclassifications	(24,192)	27,278
Amounts reclassified from accumulated other comprehensive income	14,653	(8,326)
Other comprehensive income (loss)	(9,539)	18,952
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA		14,797
Balance	7,724	83,497	49,748
Unrecognized pension and postretirement benefit obligations, net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(13,074)	(16,299)
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Other comprehensive income (loss) before reclassifications	(4,885)	5,571
Amounts reclassified from accumulated other comprehensive income	333	958
Other comprehensive income (loss)	(4,552)	6,529
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA		(3,304)
Balance	(17,626)	(13,074)	(16,299)
Unrecognized pension and postretirement benefit obligations, prior service credit
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	12,961	12,632
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Other comprehensive income (loss) before reclassifications	1,050	96
Amounts reclassified from accumulated other comprehensive income	(2,489)	(2,047)
Other comprehensive income (loss)	(1,439)	(1,951)
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA		2,280
Balance	11,522	12,961	12,632
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(113)	(3,667)
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Other comprehensive income (loss) before reclassifications	(3,835)	5,667
Amounts reclassified from accumulated other comprehensive income	(2,156)	(1,089)	(850)
Other comprehensive income (loss)	(5,991)	4,578
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA		(1,024)
Balance	(6,104)	(113)	(3,667)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	83,384	46,081
Cumulative Effect of New Accounting Principle in Period of Adoption	(66,234)
Reclassification of tax effects from accumulated other comprehensive income resulting from TCJA		13,773
Balance	$ 1,620	$ 83,384	$ 46,081