Business Acquisitions - Pro Forma Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
The Navigators Group, Inc. [Member]
Business Acquisition [Line Items]
Total Revenue	$ 21,416	$ 20,398
Net Income	$ 2,080	$ 1,828
Aetna Group Insurance [Member]
Business Acquisition [Line Items]
Total Revenue			$ 18,899
Net Income			$ (3,077)