Fair Value Measurements - Level 3 Financial Assets and Liabilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Disclosures [Abstract]
Percentage of investments in fixed maturities and equity securities classified in Level 3	3.00%
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	$ 23,726	$ 21,544
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	732	776
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	703	721
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Securities not priced by the Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	29	55
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Equity securities [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	103	74
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Private equity securities [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	$ 70	$ 74
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Private equity securities [Member] | Maximum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Multiple of earnings before interest, taxes, depreciation and amortization (EBITDA) used to value equity securities.	12	11.6
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Private equity securities [Member] | Minimum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Multiple of earnings before interest, taxes, depreciation and amortization (EBITDA) used to value equity securities.	5	4.2
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Private equity securities [Member] | Weighted Average [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Multiple of earnings before interest, taxes, depreciation and amortization (EBITDA) used to value equity securities.	8.9	8.5
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Hybrid equity securities [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	$ 33	$ 0
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Hybrid equity securities [Member] | Maximum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity adjustment to discount rates used to value fixed maturities and equity securities	2.70%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Hybrid equity securities [Member] | Minimum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity adjustment to discount rates used to value fixed maturities and equity securities	2.70%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Hybrid equity securities [Member] | Weighted Average [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity adjustment to discount rates used to value fixed maturities and equity securities	2.70%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	$ 600	647
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Corporate and government fixed maturities [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	$ 446	$ 490
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Corporate and government fixed maturities [Member] | Maximum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity adjustment to discount rates used to value fixed maturities and equity securities	16.50%	13.00%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Corporate and government fixed maturities [Member] | Minimum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity adjustment to discount rates used to value fixed maturities and equity securities	0.70%	0.80%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Corporate and government fixed maturities [Member] | Weighted Average [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity adjustment to discount rates used to value fixed maturities and equity securities	3.00%	3.40%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Mortgage and other asset-backed securities [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	$ 154	$ 157
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Mortgage and other asset-backed securities [Member] | Maximum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity adjustment to discount rates used to value fixed maturities and equity securities	3.70%	3.30%
Adjustment to discount rates used to value fixed maturities and equity securities for weighting of credit spreads	2.90%	4.70%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Mortgage and other asset-backed securities [Member] | Minimum [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity adjustment to discount rates used to value fixed maturities and equity securities	0.60%	0.60%
Adjustment to discount rates used to value fixed maturities and equity securities for weighting of credit spreads	1.80%	1.60%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Mortgage and other asset-backed securities [Member] | Weighted Average [Member] | Unobservable Inputs Developed By Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Liquidity adjustment to discount rates used to value fixed maturities and equity securities	0.90%	0.90%
Adjustment to discount rates used to value fixed maturities and equity securities for weighting of credit spreads	2.30%	2.30%