Investments - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investment [Line Items]
Gross gains on fixed income securities	$ 21,000,000		$ 95,000,000
Gross loss on fixed income securities	48,000,000		242,000,000
Gross gains on fixed income and equity securities		$ 145,000,000		$ 521,000,000
Gross loss on fixed income and equity securities		36,000,000		161,000,000
Net unrealized gains related to changes in valuation of fixed income securities subsequent to impairment measurement date	195,000,000		195,000,000		$ 208,000,000
Unrealized losses related to securities with unrealized loss position less than 20% of cost or amortized cost	862,000,000		862,000,000
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	7,602,000,000		7,602,000,000		6,740,000,000
Fixed income and equity securities
Investment [Line Items]
Unrealized losses	878,000,000		878,000,000		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	770,000,000		770,000,000
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	12,000,000		12,000,000
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	92,000,000		92,000,000
Unrealized losses having loss of less than twenty percent, less than 12 months	58,000,000		58,000,000
Unrealized losses related to securities with unrealized loss position greater than or equal to 20% of cost or amortized cost	4,000,000		4,000,000
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	1,000,000		1,000,000
EMA limited partnerships
Investment [Line Items]
Limited partnership interests	5,890,000,000		5,890,000,000		5,410,000,000
Carrying value | Cost-method Investments
Investment [Line Items]
Cost method limited partnerships	1,710,000,000		1,710,000,000		$ 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	$ 8,000,000