Stockholders' Equity and Statutory Accounting Practices Combined statutory capital and surplus and net income (loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Combined Continental Casualty Companies [Member]
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 10,723	$ 11,155
Statutory net income (loss)	$ 1,148	914	$ 913
Life Company [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)		$ 37	$ 48