SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Catastrophe Exposures (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
California earthquake
Catastrophe Exposures
Catastrophe reinsurance	$ 400	$ 300
First-dollar retention	25	25
Non-California earthquake
Catastrophe Exposures
Catastrophe reinsurance	425	325
First-dollar retention	25	25
Other perils
Catastrophe Exposures
Catastrophe reinsurance	275	225
First-dollar retention	$ 25	$ 25