Stockholders' Equity and Statutory Accounting Practices Combined statutory capital and surplus and net income (loss) (Details) - Combined Continental Casualty Companies - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory Accounting Practices [Line Items]
Statutory Capital and Surplus	$ 10,726	$ 10,748
Statutory Net Income (Loss)	$ 1,029	$ 1,033	$ 1,148