LONG-TERM DEBT (Details) - USD ($)	1 Months Ended			9 Months Ended
	Aug. 31, 2016	Jul. 31, 2016	Jan. 31, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Debt Instrument [Line Items]
Proceeds from issuance of long term debt				$ 61,000,000	$ 80,000,000
Line of credit, maximum borrowing capacity				210,000,000
Long-term debt				639,900,000		$ 605,400,000
Medium-term notes | Medium-term notes, 4.60%
Debt Instrument [Line Items]
Extinguishment of Debt, Amount		$ 17,000,000.0
Debt Instrument, Interest Rate, Stated Percentage		4.60%
Medium-term notes | Medium-term notes, 5.437%
Debt Instrument [Line Items]
Extinguishment of Debt, Amount	$ 10,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	5.437%
Line of Credit | Loans payable
Debt Instrument [Line Items]
Proceeds from issuance of long term debt			$ 61,000,000.0
Interest rate (as percent)			1.37%
Line of credit, maximum borrowing capacity			$ 200,000,000.00
Long-term debt				$ 200,000,000