Investments - Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Impaired loans without valuation allowances	$ 4		$ 4		$ 4
Less: Allowances for losses on impaired loans	0		0		0
Impaired loans, net	4		4		4
Unpaid principal balance of impaired loans	5		5		$ 5
Impaired loans, average investment during the period (amortized cost)	4	$ 4	6	$ 4
Interest income recognized on impaired loans, on an accrual basis	0	0	0	0
Interest income recognized on impaired loans, on a cash basis	0	0	0	0
Interest income recognized on troubled debt restructured loans, on an accrual basis	$ 0	$ 0	$ 0	$ 0