ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at December 31, 2016			$ 46,081
Other comprehensive income before reclassifications			17,979
Amounts reclassified from accumulated other comprehensive income			(4,680)
Other comprehensive income (loss)	$ 4,945	$ 8,470	13,299	$ 19,954
Balance at June 30, 2017	59,380		59,380
Unrealized Gains and (Losses) on Available-for-sale securities
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at December 31, 2016			49,748
Other comprehensive income before reclassifications			17,979
Amounts reclassified from accumulated other comprehensive income			(4,135)
Other comprehensive income (loss)			13,844
Balance at June 30, 2017	63,592		63,592
Unrecognized pension and postretirement benefit obligations
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at December 31, 2016			(3,667)
Other comprehensive income before reclassifications			0
Amounts reclassified from accumulated other comprehensive income			(545)
Other comprehensive income (loss)			(545)
Balance at June 30, 2017	$ (4,212)		$ (4,212)