DEBT AND FINANCING ARRANGEMENTS - Credit facilities (Details) - USD ($)		12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	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								$ 500,000,000	$ 750,000,000
LOC Facility term	4 years
Letters of credit outstanding		$ 387,000,000
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
LOC Facility | Letter of Credit Facility
Line of Credit Facility [Line Items]
Maximum capacity					$ 250,000,000
Novae Syndicated Bank Facility [Member]
Line of Credit Facility [Line Items]
Interest Expense		1,000,000
Novae Syndicated Bank Facility [Member] | Letter Of Credit And Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum capacity							$ 229,350,400.0000
Senior Notes
Line of Credit Facility [Line Items]
Interest Expense		51,000,000	$ 50,000,000	$ 50,000,000
Senior Notes | 2010 AXIS Specialty Finance LLC Senior Notes
Line of Credit Facility [Line Items]
Face amount		$ 500,000,000			$ 500,000,000
Term Loan [Member] | Novae Syndicated Bank Facility [Member]
Line of Credit Facility [Line Items]
Face amount							$ 67,000,000