Fair Value Measurements - Level 3 Financial Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
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	19,323		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	818		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	765		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	45		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.	8.0		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	720		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	53	[1]	53	[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	301		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	0.80%		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	2.80%		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	419		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	3.90%		3.70%
Adjustment to discount rates used to value fixed maturities and equity securities for weighting of credit spreads	26.30%		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	1.50%		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.