INVESTMENTS - Commercial Mortgage Loans and Transitional Real Estate Loans by Key Credit Quality Indicators (Details) $ in Millions	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	$ 9,614	$ 6,946
Weighted average debt-service coverage ratio	2.38	2.45
Transitional real estate loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	$ 5,472	$ 4,394
Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	1,710	1,065
Commercial mortgage and transitional real estate loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	7,182	5,459
Loan to Value Ratio, Zero to Fifty-nine Point Nine-nine Percent | Transitional real estate loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	1,424	819
Loan to Value Ratio, Zero to Fifty-nine Point Nine-nine Percent | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	1,390	877
Loan to Value Ratio, Zero to Fifty-nine Point Nine-nine Percent | Commercial mortgage and transitional real estate loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	2,814	1,696
Loan to Value Ratio, Sixty to Sixty-nine Point Nine-nine Percent | Transitional real estate loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	1,927	1,681
Loan to Value Ratio, Sixty to Sixty-nine Point Nine-nine Percent | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	297	165
Loan to Value Ratio, Sixty to Sixty-nine Point Nine-nine Percent | Commercial mortgage and transitional real estate loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	2,224	1,846
Loan to Value Ratio, Seventy to Seventy-nine Point Nine-nine Percent | Transitional real estate loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	2,085	1,558
Loan to Value Ratio, Seventy to Seventy-nine Point Nine-nine Percent | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	23	23
Loan to Value Ratio, Seventy to Seventy-nine Point Nine-nine Percent | Commercial mortgage and transitional real estate loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	2,108	1,581
Loan to Value Ratio Eighty Percent and Above | Transitional real estate loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	36	336
Loan to Value Ratio Eighty Percent and Above | Commercial mortgage loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	0	0
Loan to Value Ratio Eighty Percent and Above | Commercial mortgage and transitional real estate loans
Financing Receivable, Credit Quality Indicator [Line Items]
Commercial mortgage and other loans, gross	$ 36	$ 336