EMPLOYEE RETIREMENT PLANS (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pension plans [Member]
Components of net periodic benefit cost [Abstract]
Service cost	$ 3,180,920	$ 2,849,900	$ 6,606,398	$ 6,219,484
Interest cost	1,917,778	2,213,713	3,828,019	4,413,838
Expected return on plan assets	(4,287,615)	(3,731,361)	(8,575,231)	(7,462,722)
Amortization of net actuarial loss	1,515,096	1,745,644	2,981,181	3,414,404
Amortization of prior service cost	12,583	72,788	25,165	145,576
Net periodic benefit cost	2,338,762	3,150,684	4,865,532	6,730,580
Company's share of net periodic pension benefit cost	725,127	970,632	1,506,658	2,070,030
Entity's share of estimated contributions to employee benefit plan	4,500,000		4,500,000
Postretirement benefit plans [Member]
Components of net periodic benefit cost [Abstract]
Service cost	1,574,979	1,537,530	3,149,958	3,075,060
Interest cost	1,542,971	1,634,210	3,085,942	3,268,420
Expected return on plan assets	(907,750)	(804,794)	(1,815,500)	(1,609,588)
Amortization of net actuarial loss	923,505	1,002,154	1,847,010	2,004,308
Amortization of prior service cost	(622,781)	(532,814)	(1,245,562)	(1,065,628)
Net periodic benefit cost	2,510,924	2,836,286	5,021,848	5,672,572
Company's share of net periodic postretirement benefit cost	727,946	821,798	1,455,892	1,643,594
Entity's share of estimated contributions to employee benefit plan	$ 1,200,000		$ 1,200,000