Debt (details) - Junior Sub Debt $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) debentures trusts	Dec. 31, 2016 USD ($)
Debt Instrument
Unamortized fair value adjustment, debt	$ 46	$ 47
Junior subordinated debt [Member]
Debt Instrument
Number of junior subordinated debentures that are all similar in nature | debentures	3
Number of separate trusts that issued preferred securities and used proceeds to purchase the Company's subordinated debentures | trusts	3
Impact of amortization of the fair value adjustment on interest expense	$ 1	$ 2
7.625% Junior subordinated debentures due December 15, 2027 [Member]
Debt Instrument
Interest rate (percent)	7.625%	7.625%
Debt, effective interest rate (percent)	6.147%	6.147%
Unamortized fair value adjustment, debt	$ 13	$ 14
8.50% Junior subordinated debentures due December 15, 2045 [Member]
Debt Instrument
Interest rate (percent)	8.50%	8.50%
Debt, effective interest rate (percent)	6.362%	6.362%
Unamortized fair value adjustment, debt	$ 15	$ 15
8.312% Junior subordinated debentures due July 1, 2046 [Member]
Debt Instrument
Interest rate (percent)	8.312%	8.312%
Debt, effective interest rate (percent)	6.362%	6.362%
Unamortized fair value adjustment, debt	$ 18	$ 18