Investments (Valuation Allowance Rollforward by Portfolio Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Mortgage Loans on Real Estate [Line Items]
Beginning Balance	$ 318	$ 305	$ 322
Provision (release)	186	48	12
Charge-offs, net of recoveries	(160)	(35)	(29)
Ending Balance	344	318	305
Commercial
Mortgage Loans on Real Estate [Line Items]
Beginning Balance	217	224	258
Provision (release)	160	12	(11)
Charge-offs, net of recoveries	(143)	(19)	(23)
Ending Balance	234	217	224
Agricultural
Mortgage Loans on Real Estate [Line Items]
Beginning Balance	42	39	44
Provision (release)	3	3	(4)
Charge-offs, net of recoveries	(1)	0	(1)
Ending Balance	44	42	39
Residential
Mortgage Loans on Real Estate [Line Items]
Beginning Balance	59	42	20
Provision (release)	23	33	27
Charge-offs, net of recoveries	(16)	(16)	(5)
Ending Balance	$ 66	$ 59	$ 42