Fair Value Measurements - Level 3 Financial Assets and Liabilities (Details) $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Fair Value Disclosures [Abstract]
Percentage of investments in fixed maturities and equity securities classified in Level 3		4.00%
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		$ 20,719	$ 19,645
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		743	726
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		711	681
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	[1]	32	45
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		$ 70	$ 69
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | 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.		11.6	11.6
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | 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.		4.2	4.2
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | 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.		8.2	8.3
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		$ 641	$ 612
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Corporate and government debt 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		$ 425	$ 285
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Corporate and government debt 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		12.20%	9.30%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Corporate and government debt 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		0.70%	0.70%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Corporate and government debt 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		3.70%	2.80%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Other asset and mortgage-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		$ 216	$ 327
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Other asset and mortgage-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		4.70%	4.40%
Adjustment to discount rates used to value fixed maturities and equity securities for weighting of credit spreads		7.50%	6.30%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Other asset and mortgage-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		1.90%	1.70%
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Other asset and mortgage-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		2.20%	2.00%
Adjustment to discount rates used to value fixed maturities and equity securities for weighting of credit spreads		2.80%	2.20%

[1] The fair values for these securities use single, unadjusted non-binding broker quotes not developed directly by the Company.