Fair Value (Nonrecurring Fair Value Measurements) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Level 3 | Mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	$ 12	$ 99
Level 3 | Other limited partnership interests
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	43	39
Level 3 | Other assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value After Measurement	0	0
Nonrecurring | Mortgage loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gains (Losses)	0	(58)
Nonrecurring | Other limited partnership interests
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gains (Losses)	(12)	(20)
Nonrecurring | Other assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gains (Losses)	$ (3)	$ (14)