Fair Value (Nonrecurring Fair Value Measurements) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Level 3 | Mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	$ 12	$ 26	$ 12	$ 26
Level 3 | Other limited partnership interests
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	61	62	61	62
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)	0	(85)	0	(143)
Nonrecurring | Other limited partnership interests
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gains (Losses)	(12)	(16)	(24)	(36)
Nonrecurring | Other assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gains (Losses)	$ (2)	$ (30)	$ (5)	$ (44)