ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	$ 59,010	[1]	$ 47,752	[1]
Other comprehensive income (loss) before reclassifications	29,103	[1]	15,684	[1]
Amounts reclassified from accumulated other comprehensive income	(6,451)	[1]	(4,426)	[1]
Other comprehensive income	22,652	[1]	11,258	[1]	20,412
Balance at end of period	81,662	[1]	59,010	[1]	47,752	[1]
Unrealized Gains and (Losses) on Available-for-sale securities
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	48,376	[1]	69,806	[1]
Other comprehensive income (loss) before reclassifications	34,663	[1]	(15,582)	[1]
Amounts reclassified from accumulated other comprehensive income	(4,677)	[1]	(5,848)	[1]
Other comprehensive income	29,986	[1]	(21,430)	[1]
Balance at end of period	78,362	[1]	48,376	[1]
Unrecognized pension and postretirement benefit obligations
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	10,634	[1]	(22,054)	[1]
Other comprehensive income (loss) before reclassifications	(5,560)	[1]	31,266	[1]
Amounts reclassified from accumulated other comprehensive income	(1,774)	[1]	1,422	[1]
Other comprehensive income	(7,334)	[1]	32,688	[1]
Balance at end of period	$ 3,300	[1]	$ 10,634	[1]

[1] All amounts are net of tax. Amounts in parentheses indicate debits.