DEBT AND FINANCING ARRANGEMENTS - Credit facilities (Details) - USD ($)		12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2018	Mar. 28, 2018	Mar. 27, 2017	Mar. 26, 2017	Aug. 02, 2016	Mar. 31, 2015	Mar. 30, 2015
Letter of Credit Facility | Citibank
Line of Credit Facility [Line Items]
Maximum capacity				$ 750,000,000				$ 500,000,000	$ 750,000,000
LOC Facility term	4 years
Letters of credit outstanding			$ 395,000,000
Revolving Credit Facility | Syndication Of Lenders
Line of Credit Facility [Line Items]
Potential increase in maximum capacity			$ 150,000,000
Credit Agreement | Letter of Credit Facility
Line of Credit Facility [Line Items]
Maximum capacity						$ 250,000,000
$500 million secured LOC | Letter of Credit Facility | Citibank
Line of Credit Facility [Line Items]
Maximum capacity				$ 500,000,000	$ 500,000,000
$250 million secured LOC | Letter of Credit Facility | Citibank
Line of Credit Facility [Line Items]
Maximum capacity					$ 250,000,000
Novae Syndicated Bank Facility
Line of Credit Facility [Line Items]
Interest Expense		$ 1,000,000
Novae Syndicated Bank Facility | Letter Of Credit And Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum capacity							$ 229,000,000
Term Loan | Novae Syndicated Bank Facility
Line of Credit Facility [Line Items]
Face amount							$ 67,000,000