Investments Investments - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Fair value of assets and liabilities measured on recurring and non-recurring basis
Gross gains on sales of fixed income securities, debt	$ 13,000,000
Gross gains on sales of fixed income securities		$ 7,000,000
Gross losses on sales of fixed income securities, debt	(19,000,000)	(12,000,000)
Net unrealized gains related to changes in valuation of fixed income securities subsequent to impairment measurement date	100,000,000		$ 101,000,000
Unrealized losses related to securities with unrealized loss position less than 20% of amortized cost or cost	(107,000,000)
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	(22,000,000)
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost, greater than 12 months	1,000,000
Limited partnership interests	3,219,000,000		3,292,000,000
Short-term investments	1,284,000,000		810,000,000
Other	1,348,000,000		1,300,000,000
Fixed income securities
Fair value of assets and liabilities measured on recurring and non-recurring basis
Total unrealized losses	129,000,000		389,000,000
Fixed income and equity securities
Fair value of assets and liabilities measured on recurring and non-recurring basis
Total unrealized losses	129,000,000
Investment grade fixed income securities
Fair value of assets and liabilities measured on recurring and non-recurring basis
Unrealized losses related to securities with unrealized loss position less than 20% of amortized cost or cost	(81,000,000)
Total unrealized losses	93,000,000		268,000,000
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	(12,000,000)
Below investment grade equity securities
Fair value of assets and liabilities measured on recurring and non-recurring basis
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of amortized cost or cost	(10,000,000)
Below investment grade fixed income securities
Fair value of assets and liabilities measured on recurring and non-recurring basis
Unrealized losses related to securities with unrealized loss position less than 20% of amortized cost or cost	(26,000,000)
Total unrealized losses	36,000,000		121,000,000
Available for sale securities, unrealized losses having loss of less than twenty percent, less than 12 months	10,000,000
Equity method limited partnerships
Fair value of assets and liabilities measured on recurring and non-recurring basis
Limited partnership interests	2,460,000,000		2,510,000,000
Carrying value | Cost method limited partnership interests
Fair value of assets and liabilities measured on recurring and non-recurring basis
Limited partnership interests	758,000,000		$ 787,000,000
Non-impaired mortgage loans
Fair value of assets and liabilities measured on recurring and non-recurring basis
Allowance for credit losses, period increase (decrease)	0	0
Mortgage loans
Fair value of assets and liabilities measured on recurring and non-recurring basis
Average balance impaired mortgage loans	$ 4,000,000	$ 4,000,000