Basis of Presentation and Significant Accounting Policies (Details) - USD ($)	May 23, 2019	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2018
Accounting Standards Update 2017-04 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets		$ 0
Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			$ (2,000,000)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets				$ 0
Retained Earnings | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			$ (18,000,000)
The Navigators Group, Inc. [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Business Acquisition, Share Price	$ 70
Business Combination, Consideration Transferred	$ 2,137,000,000