ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at December 31, 2015			$ 58,433
Other comprehensive income before reclassifications			20,698
Amounts reclassified from accumulated other comprehensive income			(3,602)
Other comprehensive income (loss)	$ (2,858)	$ (4,729)	17,096	$ (20,858)
Balance at September 30, 2016	75,529		75,529
Unrealized Gains and (Losses) on Available-for-sale securities
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at December 31, 2015			60,369
Other comprehensive income before reclassifications			20,698
Amounts reclassified from accumulated other comprehensive income			(2,995)
Other comprehensive income (loss)			17,703
Balance at September 30, 2016	78,072		78,072
Unrecognized pension and postretirement benefit obligations
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at December 31, 2015			(1,936)
Other comprehensive income before reclassifications			0
Amounts reclassified from accumulated other comprehensive income			(607)
Other comprehensive income (loss)			(607)
Balance at September 30, 2016	$ (2,543)		$ (2,543)