Investments (Valuation Allowance Rollforward by Portfolio Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	$ 344	$ 318
Provision (release)	13	68
Charge-offs, net of recoveries	(4)	(5)
Balance, end of period	353	381
Commercial
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	234	217
Provision (release)	6	64
Charge-offs, net of recoveries	0	0
Balance, end of period	240	281
Agricultural
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	44	42
Provision (release)	1	0
Charge-offs, net of recoveries	0	(2)
Balance, end of period	45	40
Residential
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	66	59
Provision (release)	6	4
Charge-offs, net of recoveries	(4)	(3)
Balance, end of period	$ 68	$ 60