DEBT AND CREDIT AGREEMENTS (Long-term Debt) (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total Long-Term Debt	$ 1,614,000	$ 1,629,000
Less Unamortized Discount and Debt Issuance Costs	12,000	14,000
Current Maturities of Long-Term Debt	80,000	0
Total Long-Term Debt, Net	$ 1,522,087	1,615,252
2011 Notes | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate	5.15%
Total Long-Term Debt	$ 250,000	250,000
2012 Notes | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate	3.85%
Total Long-Term Debt	$ 150,000	150,000
2014 Notes | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate	5.00%
Total Long-Term Debt	$ 150,000	150,000
2015 Notes | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate	3.05%
Total Long-Term Debt	$ 300,000	300,000
2018 Notes | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate	4.85%
Total Long-Term Debt	$ 300,000	300,000
2010 Pima A | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate	5.25%
Total Long-Term Debt	$ 100,000	100,000
2012 Pima A | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate	4.50%
Total Long-Term Debt	$ 16,000	16,000
2013 Pima A | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate	4.00%
Total Long-Term Debt	$ 91,000	91,000
2009 Pima A | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate	4.95%
Total Long-Term Debt	$ 80,000	80,000
2009 Coconino A | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate	5.13%
Total Long-Term Debt	$ 0	15,000
2012 Apache A | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate	4.50%
Total Long-Term Debt	$ 177,000	$ 177,000