Investments (Valuation Allowance Rollforward by Portfolio Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	$ 314	$ 304
Provision (release)	14	10
Charge-offs, net of recoveries	(1)	(4)
Balance, end of period	327	310
Commercial
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	214	202
Provision (release)	14	5
Charge-offs, net of recoveries	0	0
Balance, end of period	228	207
Agricultural
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	41	39
Provision (release)	0	0
Charge-offs, net of recoveries	0	0
Balance, end of period	41	39
Residential
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	59	63
Provision (release)	0	5
Charge-offs, net of recoveries	(1)	(4)
Balance, end of period	$ 58	$ 64