Investments (Valuation Allowance Rollforward by Portfolio Segment) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	$ 304	$ 281
Provision (release) (1)	25	163
Charge-offs, net of recoveries (1)	(13)	(157)
Balance, end of period	316	287
Commercial
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	202	188
Provision (release) (1)	11	149
Charge-offs, net of recoveries (1)	0	(143)
Balance, end of period	213	194
Agricultural
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	39	37
Provision (release) (1)	4	3
Charge-offs, net of recoveries (1)	(2)	(2)
Balance, end of period	41	38
Residential
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	63	56
Provision (release) (1)	10	11
Charge-offs, net of recoveries (1)	(11)	(12)
Balance, end of period	$ 62	$ 55