Investments (Valuation Allowance Rollforward by Portfolio Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate [Line Items]
Beginning Balance	$ 304	$ 281	$ 279
Provision (release)	24	183	37
Charge-offs, net of recoveries	(14)	(160)	(35)
Ending Balance	314	304	281
Commercial
Mortgage Loans on Real Estate [Line Items]
Beginning Balance	202	188	202
Provision (release)	12	157	5
Charge-offs, net of recoveries	0	(143)	(19)
Ending Balance	214	202	188
Agricultural
Mortgage Loans on Real Estate [Line Items]
Beginning Balance	39	37	35
Provision (release)	4	3	2
Charge-offs, net of recoveries	(2)	(1)	0
Ending Balance	41	39	37
Residential
Mortgage Loans on Real Estate [Line Items]
Beginning Balance	63	56	42
Provision (release)	8	23	30
Charge-offs, net of recoveries	(12)	(16)	(16)
Ending Balance	$ 59	$ 63	$ 56