Schedule II - Condensed Financial Information, Credit Facility (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Debt outstanding	$ 5,200
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit agreement term	5 years
Maximum borrowing capacity under credit facility	$ 1,500
Number of participating banks	16
Total commitment amount under option to increase borrowing capacity	$ 2,000
Expiration date	Dec. 12, 2019
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 that is included in accumulated other comprehensive loss on the Company’s consolidated balance sheets.
Leverage ratio covenant	50.00%
Additional borrowing capacity under maximum debt coverage covenant	$ 8,300
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facility	500
Letters of credit outstanding	$ 14
Line of Credit Facility, Covenant Compliance	The Company was in compliance with its debt covenants as of March 31, 2016.
Primary Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Number of participating banks	3
Percent held by each primary commitment holder	12.00%
Minor Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Number of participating banks	13
Collective percentage held by secondary commitment holders	64.00%