Investments (Narrative) (Details) $ in Millions	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Gain (Loss) on Investments
Largest equity holdings to total equity portfolio	4.00%
Largest equity holdings to total portfolio	1.40%
Investments in fair value, deposit with various states	$ 17,774	$ 16,193
Impaired securities	6	4	20
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss	$ 18	$ 11	$ 37
Collateral at fair value	$ 57	$ 22
Fixed Maturities
Gain (Loss) on Investments
Number of securities total unrealized loss in an unrealized loss position for 12 months or more	249	32	69
Number of securities below 70 percent of amortized cost	0	0	5
Equity Securities
Gain (Loss) on Investments
Number of securities total unrealized loss in an unrealized loss position for 12 months or more	0	0	0
Exxon Mobil Corporation, The Procter & Gamble Company, Honeywell International Inc, BlackRock Inc., Genuine Parts Company, RPM International, and Johnson and Johnson [Member]
Gain (Loss) on Investments
Available-for-sale securities, accumulated net gain position	$ 1,013
Seven Primary Contributors
Gain (Loss) on Investments
Number of securities	7
On Deposit With Various States
Gain (Loss) on Investments
Investments in fair value, deposit with various states	$ 101	$ 72