Debt - Revolving Credit and Letter of Credit (Details) - Jun. 30, 2015 $ in Millions	USD ($)
Line of Credit Facility [Line Items]
Short-term and Long-term Debt Outstanding	$ 5,200
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Dec. 12, 2019
Line of Credit Facility, Term of Agreement	5 years
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,500
Line of Credit Facility, Number of Participating Banks	16
Line of Credit Facility, Optional Maximum Borrowing Capacity	$ 2,000
Line of Credit Facility, Covenant Terms	The credit agreement includes options subject to consent by the administrative agent and the committing banks to increase the commitment amount to $2 billion and to extend the term past December 12, 2019. The credit agreement is available for general corporate purposes, including for the issuance of letters of credit. The credit agreement contains customary covenants and restrictions, including a financial covenant that the Company may not permit its leverage ratio – which is total consolidated debt to total consolidated capitalization (each as defined in the credit agreement) – to be greater than 0.50. The leverage ratio calculation excludes net unrealized appreciation in fixed maturities and the portion of the post-retirement benefits liability adjustment attributable to pension as included in accumulated other comprehensive loss on the Company’s consolidated balance sheets.
Maximum consolidated debt-to-consolidated capitalization ratio convenant	50.00%
Credit Facility Remaining Borrowing Capacity	$ 7,100
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	500
Letters of Credit Outstanding, Amount	$ 19
Line of Credit Facility, Covenant Compliance	The Company was in compliance with its debt covenants as of June 30, 2015.
Primary Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Number of Participating Banks	3
Line of Credit Facility, Individual Bank Percentage Committment	12.00%
Minor Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Number of Participating Banks	13
Line of Credit Facility, Collective Bank Percentage Committment	64.00%