Business Acquisitions - Fair Value of Assets Acquired and Liabilities Assumed at the Acquisition Date (Details) - USD ($) $ in Millions	Sep. 30, 2019	May 23, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 1,913		$ 1,290
The Navigators Group, Inc. [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents		$ 3,848
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Premiums Receivable		492
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Reinsurance Recoverables		1,100
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Prepaid Reinsurance Premiums		238
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill		580
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment		83
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets		99
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets		6,440
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Unpaid Losses and Loss Adjustment Expenses		2,823
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Deferred Revenue		1,219
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt		284
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets		48
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other		568
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities		4,942
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net		1,498
Goodwill		623
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net		$ 2,121