Fair Value Measurements - Level 3 Financial Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Fair Value Disclosures [Abstract]
Percentage of investments in fixed maturities and equity securities classified in Level 3	5.00%
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	19,172		16,627
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	857		1,190
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	804		1,068
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	43		59
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.	8.1		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	761		1,009
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	53	[1]	122	[1]
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	344		416
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	0.80%		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	2.62%		3.10%
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	417		593
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	3.70%		6.20%
Adjustment to discount rates used to value fixed maturities and equity securities for weighting of credit spreads	25.60%		20.90%
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	1.60%		1.20%
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	1.40%		1.70%
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.