EMPLOYEE BENEFITS (Net Periodic Benefit Cost) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plan [Member]
Net periodic benefit cost
Service cost	$ 5,210	$ 6,300	$ 5,129
Interest cost	5,874	5,175	5,049
Expected return on plan assets	(6,956)	(5,772)	(5,345)
Amortization of prior service cost	0	0	8
Amortization of net loss	2,174	4,989	4,415
Net periodic benefit cost	6,302	10,692	9,256
Pension Plan [Member] | Mercer Insurance Group [Member]
Net periodic benefit cost
Net periodic benefit cost			854
Postretirement Benefit Plan [Member]
Net periodic benefit cost
Service cost	3,696	3,875	3,010
Interest cost	2,342	1,759	1,695
Expected return on plan assets	0	0	0
Amortization of prior service cost	0	0	(6)
Amortization of net loss	898	879	554
Net periodic benefit cost	$ 6,936	$ 6,513	$ 5,253