DEBT, CREDIT FACILITY, AND CAPITAL LEASE OBLIGATIONS (Long-term Debt) (Detail) - USD ($)	1 Months Ended
	Aug. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Feb. 28, 2015	Jan. 31, 2015
Debt Instrument [Line Items]
Total Long-Term Debt		$ 1,466,000,000	$ 1,466,000,000
Less Unamortized Discount and Debt Issuance Costs		12,000,000	13,000,000
Current Maturities of Long-Term Debt		100,000,000	0
Total Long-Term Debt, Net		$ 1,354,423,000	1,453,072,000
Notes 2011 5.15% due 2021 | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate (in percentage)		5.15%
Total Long-Term Debt		$ 250,000,000	250,000,000
Notes 2012 3.85% due 2023 | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate (in percentage)		3.85%
Total Long-Term Debt		$ 150,000,000	150,000,000
Notes 2014 5.00% due 2044 | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate (in percentage)		5.00%
Total Long-Term Debt		$ 150,000,000	150,000,000
Notes 2015 3.05% due 2025 | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate (in percentage)		3.05%
Total Long-Term Debt		$ 300,000,000	300,000,000
Debt instrument, face amount				$ 300,000,000
Tax Exempt Local Furnishings Bonds 2010 Pima A due 2040 5.25% | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate (in percentage)		5.25%
Total Long-Term Debt		$ 100,000,000	100,000,000
Tax Exempt Local Furnishings Bonds 2012 Pima A due 2030 4.50% | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate (in percentage)		4.50%
Total Long-Term Debt		$ 16,000,000	16,000,000
Tax Exempt Local Furnishings Bonds 2013 Pima A due 2029 4.00% | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate (in percentage)		4.00%
Total Long-Term Debt		$ 91,000,000	91,000,000
Tax Exempt Local Furnishings Bonds 2013 Apache A due 2032 Variable Rate | Unsecured Debt
Debt Instrument [Line Items]
Effective interest rate (in percentage)		1.41%
Total Long-Term Debt		$ 100,000,000	100,000,000
Debt instrument, face amount		$ 100,000,000
Tax Exempt Pollution Control Bonds 2009 Pima A due 2020 4.95% | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate (in percentage)		4.95%
Total Long-Term Debt		$ 80,000,000	80,000,000
Tax Exempt Pollution Control Bonds 2009 Coconino A due 2032 5.13% | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate (in percentage)		5.13%
Total Long-Term Debt		$ 15,000,000	15,000,000
Tax Exempt Pollution Control Bonds 2010 Coconino A due 2032 Variable Rate | Secured Debt
Debt Instrument [Line Items]
Effective interest rate (in percentage)		1.76%
Total Long-Term Debt		$ 37,000,000	37,000,000
Debt instrument, face amount		$ 37,000,000
Tax Exempt Pollution Control Bonds 2012 Apache A due 2030 4.50% | Unsecured Debt
Debt Instrument [Line Items]
Interest Rate (in percentage)		4.50%
Total Long-Term Debt		$ 177,000,000	$ 177,000,000
Tax Exempt Local Furnishings Bonds 2008 Pima B 5.75% due 2029 | Unsecured Debt
Debt Instrument [Line Items]
Debt instrument, repurchased face amount					$ 130,000,000
Tax Exempt Variable Rate | Secured Debt
Debt Instrument [Line Items]
Debt extinguishment	$ 79,000,000