Basis of Presentation and Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Millions	May 23, 2019	Jan. 01, 2019	Sep. 30, 2019	Jul. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect on Retained Earnings, before Tax		$ 1
Operating Leases, Future Minimum Payments Due		160
Operating Lease, Right-of-Use Asset		150	$ 200	$ 34
Lease Incentive Receivable		$ 10
The Navigators Group, Inc. [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Business Acquisition, Share Price	$ 70
Business Combination, Consideration Transferred	$ 2,137