Debt - Revolving Credit and Letter of Credit (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Banks
Line of Credit Facility [Line Items]
Debt outstanding	$ 5,000
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. 31, 2019
Number of participating banks | Banks	16
Maximum borrowing capacity under option to increase	$ 2,000
Covenant terms	The credit agreement contains customary covenants and restrictions, including a financial covenant that the Company may not permit its leverage ratio – that is total consolidated debt to total consolidated capitalization (each as defined in the credit agreement) – to exceed 0.50. The leverage ratio excludes the following items that are included in accumulated other comprehensive loss on the Company’s Consolidated Balance Sheets: net unrealized appreciation in fixed maturities and the portion of the post-retirement benefits liability adjustment attributable to pension.
Leverage ratio covenant	50.00%
Debt covenant compliance	The Company was in compliance with its debt covenants as of December 31, 2016.
Borrowing capacity within maximum debt coverage covenant	$ 9,700
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facility	500
Letters of credit outstanding	$ 14
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%