Investments (Valuation Allowance Rollforward by Portfolio Segment) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	$ 318	$ 305
Provision (release) (1)	169	32
Charge-offs, net of recoveries (1)	(157)	(26)
Balance, end of period	330	311
Commercial
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	217	224
Provision (release) (1)	153	3
Charge-offs, net of recoveries (1)	(143)	(12)
Balance, end of period	227	215
Agricultural
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	42	39
Provision (release) (1)	3	2
Charge-offs, net of recoveries (1)	(2)	0
Balance, end of period	43	41
Residential
Mortgage Loans on Real Estate [Line Items]
Balance, beginning of period	59	42
Provision (release) (1)	13	27
Charge-offs, net of recoveries (1)	(12)	(14)
Balance, end of period	$ 60	$ 55