Fair Value (Nonrecurring Fair Value Measurements) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Level 3 | Mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	$ 19	$ 9	$ 19	$ 9
Level 3 | Other limited partnership interests
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	85	75	85	75
Level 3 | Other assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	0	0	0	0
Nonrecurring | Mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gains (Losses)	(1)	0	(1)	0
Nonrecurring | Other limited partnership interests
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gains (Losses)	(30)	(9)	(54)	(43)
Nonrecurring | Other assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gains (Losses)	$ 0	$ 0	$ (5)	$ (30)