Schedule II - Condensed Financial Information, Credit Facility (Details) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($) Banks
Line of Credit Facility [Line Items]
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 – 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 the following items that are included in accumulated other comprehensive loss on the Company’s consolidated balance sheets: net unrealized appreciation on fixed maturities and the portion of the post-retirement benefits liability adjustment attributable to pension.
Debt covenant compliance	The Company was in compliance with its debt covenants as of June 30, 2017.
Debt outstanding	$ 4,800
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
Expiration date	Dec. 12, 2019
Number of participating banks | Banks	16
Maximum borrowing capacity under option to increase	$ 2,000
Leverage ratio covenant	50.00%
Borrowing capacity within maximum debt coverage covenant	$ 10,700
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facility	500
Letters of credit outstanding	$ 13
Primary Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks	3
Percent of commitment 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 | Banks	13
Collective percentage held by remaining commitment holders	64.00%