DEBT - Credit Agreement (Details)	3 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Actual net worth	$ 10,081,000,000	$ 9,842,000,000
Credit Agreement | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Term	5 years
Maximum borrowing capacity	$ 2,500,000,000		$ 1,000,000,000.0
Minimum level of net worth required	9,500,000,000
Actual net worth	$ 10,100,000,000
Required maximum leverage ratio	3.0
Actual leverage ratio	1.4
Uncommitted incremental loan facility	$ 500,000,000
Line of credit, outstanding borrowings	0
Credit Agreement | Line of Credit | Letter of Credit
Line of Credit Facility [Line Items]
Line of credit, outstanding borrowings	$ 0
Credit Agreement | London Interbank Offered Rate (LIBOR) | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Interest rate spread	1.10%
Facility fee (percentage)	0.15%
Credit Agreement | Minimum | London Interbank Offered Rate (LIBOR) | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Interest rate spread	0.91%
Facility fee (percentage)	0.09%
Credit Agreement | Maximum | London Interbank Offered Rate (LIBOR) | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Interest rate spread	1.50%
Facility fee (percentage)	0.25%
Unsecured Revolving Credit Agreement, Expires May 2022 | Line of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Term	5 years
Maximum borrowing capacity			$ 2,000,000,000
Remaining borrowing capacity	$ 2,000,000,000