Schedule II - Condensed Financial Information, Short-term and Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 5,191	$ 5,199
Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	5,191	5,199
$78 million, 6.37% Notes due 2021 [Member] | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, carrying value	$ 78	$ 78