ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	$ 81,662	$ 59,010
Other comprehensive income (loss) before reclassifications	(16,986)	29,103
Amounts reclassified from accumulated other comprehensive income	(6,243)	(6,451)
Other comprehensive income (loss)	(23,229)	22,652	$ 11,258
Balance at end of period	58,433	81,662	59,010
Unrealized Gains and (Losses) on Available-for-sale securities
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	78,362	48,376
Other comprehensive income (loss) before reclassifications	(13,037)	34,663
Amounts reclassified from accumulated other comprehensive income	(4,956)	(4,677)
Other comprehensive income (loss)	(17,993)	29,986
Balance at end of period	60,369	78,362	48,376
Unrecognized pension and postretirement benefit obligations
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	3,300	10,634
Other comprehensive income (loss) before reclassifications	(3,949)	(5,560)
Amounts reclassified from accumulated other comprehensive income	(1,287)	(1,774)
Other comprehensive income (loss)	(5,236)	(7,334)
Balance at end of period	$ (1,936)	$ 3,300	$ 10,634