Investments - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Investment [Line Items]
Gross gains on fixed income securities	$ 29,000,000		$ 74,000,000
Gross loss on fixed income securities	107,000,000		194,000,000
Gross gains on fixed income and equity securities		$ 141,000,000		$ 376,000,000
Gross loss on fixed income and equity securities		50,000,000		125,000,000
Net unrealized gains related to changes in valuation of fixed income securities subsequent to impairment measurement date	197,000,000		197,000,000		$ 208,000,000
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	851,000,000		851,000,000
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	16,000,000		16,000,000
Limited partnership interests	7,679,000,000		7,679,000,000		6,740,000,000
Fixed income and equity securities
Investment [Line Items]
Unrealized losses	867,000,000		867,000,000		295,000,000
Investment grade fixed income securities
Investment [Line Items]
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	730,000,000		730,000,000
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	11,000,000		11,000,000
Below investment grade fixed income securities
Investment [Line Items]
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	121,000,000		121,000,000
Unrealized losses having loss of less than twenty percent, less than 12 months	107,000,000		107,000,000
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	5,000,000		5,000,000
Unrealized losses related to securities with unrealized loss position greater than 20% of cost or amortized cost, unrealized loss position of 12 or more consecutive months	3,000,000		3,000,000
EMA limited partnerships
Investment [Line Items]
Limited partnership interests	6,030,000,000		6,030,000,000		5,410,000,000
Carrying value | Cost-method Investments
Investment [Line Items]
Cost method limited partnerships	1,650,000,000		1,650,000,000		$ 1,330,000,000
Mortgage loans, non-impaired
Investment [Line Items]
Allowance for credit losses, period increase (decrease)	$ 0	$ 0	0	0
Average impaired mortgage loans			$ 4,000,000	$ 5,000,000