LOANS AND ALLOWANCE FOR LOAN LOSSES (Activity in Allowance for Loan Losses by Portfolio Segment) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Reserve for unfunded commitments	$ 23,000	$ 9,000	$ 20,000
Activity in ALL:
Beginning balance	24,171,000	23,116,000	23,116,000
Loans charged off	(803,000)	(159,000)	(2,468,000)
Recoveries	122,000	183,000	497,000
Provision (credit)(1)	(500,000)	581,000	3,026,000
Ending balance	22,990,000	23,721,000	24,171,000
Ending Balance: Individually evaluated for impairment	1,033,000	1,734,000	2,009,000
Ending Balance: Collectively evaluated for impairment	21,957,000	21,987,000	22,162,000
Ending Balance: Individually evaluated for impairment	11,225,000	20,128,000	13,590,000
Ending Balance: Collectively evaluated for impairment	2,777,923,000	2,625,011,000	2,768,849,000
Total Loans Outstanding	2,789,148,000	2,645,139,000	2,782,439,000
Residential Real Estate
Activity in ALL:
Beginning balance	5,086,000	4,160,000	4,160,000
Loans charged off	(31,000)	(5,000)	(482,000)
Recoveries	0	0	30,000
Provision (credit)(1)	442,000	116,000	1,378,000
Ending balance	5,497,000	4,271,000	5,086,000
Ending Balance: Individually evaluated for impairment	553,000	485,000	568,000
Ending Balance: Collectively evaluated for impairment	4,944,000	3,786,000	4,518,000
Ending Balance: Individually evaluated for impairment	5,059,000	4,408,000	5,171,000
Ending Balance: Collectively evaluated for impairment	855,474,000	815,231,000	853,198,000
Total Loans Outstanding	860,533,000	819,639,000	858,369,000
Commercial Real Estate
Activity in ALL:
Beginning balance	11,863,000	12,154,000	12,154,000
Loans charged off	(426,000)	(3,000)	(124,000)
Recoveries	13,000	103,000	141,000
Provision (credit)(1)	(1,164,000)	472,000	(308,000)
Ending balance	10,286,000	12,726,000	11,863,000
Ending Balance: Individually evaluated for impairment	368,000	1,100,000	1,441,000
Ending Balance: Collectively evaluated for impairment	9,918,000	11,626,000	10,422,000
Ending Balance: Individually evaluated for impairment	3,961,000	13,191,000	6,199,000
Ending Balance: Collectively evaluated for impairment	1,165,572,000	1,083,284,000	1,157,824,000
Total Loans Outstanding	1,169,533,000	1,096,475,000	1,164,023,000
Commercial
Activity in ALL:
Beginning balance	4,171,000	3,755,000	3,755,000
Loans charged off	(171,000)	(136,000)	(1,014,000)
Recoveries	63,000	77,000	301,000
Provision (credit)(1)	63,000	119,000	1,129,000
Ending balance	4,126,000	3,815,000	4,171,000
Ending Balance: Individually evaluated for impairment	0	0
Ending Balance: Collectively evaluated for impairment	4,126,000	3,815,000	4,171,000
Ending Balance: Individually evaluated for impairment	1,714,000	1,994,000	1,791,000
Ending Balance: Collectively evaluated for impairment	376,301,000	331,613,000	371,609,000
Total Loans Outstanding	378,015,000	333,607,000	373,400,000
Home Equity
Activity in ALL:
Beginning balance	2,367,000	2,194,000	2,194,000
Loans charged off	(149,000)	(1,000)	(434,000)
Recoveries	43,000	1,000	2,000
Provision (credit)(1)	166,000	(87,000)	605,000
Ending balance	2,427,000	2,107,000	2,367,000
Ending Balance: Individually evaluated for impairment	112,000	83,000
Ending Balance: Collectively evaluated for impairment	2,315,000	2,024,000	2,367,000
Ending Balance: Individually evaluated for impairment	491,000	430,000	429,000
Ending Balance: Collectively evaluated for impairment	320,151,000	322,396,000	322,949,000
Total Loans Outstanding	320,642,000	322,826,000	323,378,000
Consumer
Activity in ALL:
Beginning balance	233,000	181,000	181,000
Loans charged off	(26,000)	(14,000)	(124,000)
Recoveries	3,000	2,000	17,000
Provision (credit)(1)	20,000	6,000	159,000
Ending balance	230,000	175,000	233,000
Ending Balance: Individually evaluated for impairment	0	0
Ending Balance: Collectively evaluated for impairment	230,000	175,000	233,000
Ending Balance: Individually evaluated for impairment	0	7,000
Ending Balance: Collectively evaluated for impairment	18,011,000	16,662,000	18,149,000
Total Loans Outstanding	18,011,000	16,669,000	18,149,000
HPFC Portfolio Segment
Activity in ALL:
Beginning balance	451,000	672,000	672,000
Loans charged off	0	0	(290,000)
Recoveries	0	0	6,000
Provision (credit)(1)	(27,000)	(45,000)	63,000
Ending balance	424,000	627,000	451,000
Ending Balance: Individually evaluated for impairment	0	66,000
Ending Balance: Collectively evaluated for impairment	424,000	561,000	451,000
Ending Balance: Individually evaluated for impairment	0	98,000
Ending Balance: Collectively evaluated for impairment	42,414,000	55,825,000	45,120,000
Total Loans Outstanding	$ 42,414,000	$ 55,923,000	$ 45,120,000