Basis of Presentation and Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended
	Jan. 01, 2019	Aug. 22, 2018	Mar. 31, 2019	Dec. 31, 2018
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		$ 157
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		$ 2,200
The Navigators Group, Inc. [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amount Exceeding Recorded Value				$ 100
Reinsurance Retention Policy, Amount Retained				$ 1,816
The Navigators Group, Inc. [Member] | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured			300
Reinsurance Retention Policy, Amount Retained			$ 1,916