Investments (Valuation Allowance Rollforward by Portfolio Segment) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	$ 344	$ 318
Provision (release)	20	158
Charge-offs, net of recoveries	(8)	(9)
Balance, end of period	356	467
Commercial
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	234	217
Provision (release)	8	150
Charge-offs, net of recoveries	0	0
Balance, end of period	242	367
Agricultural
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	44	42
Provision (release)	2	1
Charge-offs, net of recoveries	0	(2)
Balance, end of period	46	41
Residential
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	66	59
Provision (release)	10	7
Charge-offs, net of recoveries	(8)	(7)
Balance, end of period	$ 68	$ 59