Fair Value Measurements - Level 3 Financial Assets and Liabilities (Details) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
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		$ 18,931	$ 19,172
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		721	857
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		672	804
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		$ 52	$ 43
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.		9.8	9.8
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.		7.9	8.1
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		$ 620	$ 761
Fair Value, Measurements, Recurring [Member] | Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | Pricing Exemption Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Fixed Maturities And Equity Securities	[1]	49	53
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		$ 281	$ 344
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		9.30%	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%	0.80%
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		2.60%	2.62%
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		$ 339	$ 417
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		4.30%	3.70%
Adjustment to discount rates used to value fixed maturities and equity securities for weighting of credit spreads		19.50%	25.60%
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		1.70%	1.60%
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		2.10%	1.40%
Adjustment to discount rates used to value fixed maturities and equity securities for weighting of credit spreads		2.90%	2.90%

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