Investments - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Investment [Line Items]
Gross gains on fixed income securities	$ 126,000,000
Gross loss on fixed income securities	60,000,000
Gross gains on fixed income and equity securities		$ 45,000,000
Gross loss on fixed income and equity securities		(87,000,000)
Net unrealized gains related to changes in valuation of fixed income securities subsequent to impairment measurement date	178,000,000		$ 180,000,000
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	(231,000,000)
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	(33,000,000)
Limited partnership interests	7,493,000,000		7,505,000,000
Short-term investments	4,157,000,000		3,027,000,000
Fixed income and equity securities
Investment [Line Items]
Unrealized losses	(264,000,000)
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	(179,000,000)
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	(13,000,000)
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	(52,000,000)
Unrealized losses having loss of less than twenty percent, less than 12 months	21,000,000
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	(20,000,000)
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	2,000,000
EMA limited partnerships
Investment [Line Items]
Limited partnership interests	5,760,000,000		5,730,000,000
Carrying value | Cost-method Investments
Investment [Line Items]
Cost method limited partnerships	1,740,000,000		$ 1,780,000,000
Mortgage loans, non-impaired
Investment [Line Items]
Allowance for credit losses, period increase (decrease)	0	0
Average impaired mortgage loans	$ 4,000,000	$ 4,000,000