Business Acquisitions - Fair Value of Assets Acquired and Liabilities Assumed at the Acquisition Date (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2019	May 23, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 01, 2017
Business Acquisition [Line Items]
Goodwill			$ 1,913		$ 1,290	$ 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 Liabilities				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
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles				$ 499
Aetna Group Insurance [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents					3,405		$ 3,360
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Premiums Receivable					103		96
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Reinsurance Recoverables					31		0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment					68		68
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets					0		16
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets					4,305		4,225
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Unpaid Losses and Loss Adjustment Expenses					2,904		2,833
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Deferred Revenue					4		3
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other					75		69
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities					3,576		3,496
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net					729		729
Goodwill					723		723
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net					1,452		1,452
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Assets		$ 45
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Premiums Receivable		7
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Assets					69		56
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Income Taxes		13
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles					629		629
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Intangibles		0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, and Equipment		0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Reinsurance Recoverables		31
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Assets		(16)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Total Assets	$ 80	80
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Unpaid Loss and Loss Adjustment Expenses		71
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed,Liability for Future Policy Benefits					347		346
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Liability for Future Policy Benefits		1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Policyholder Benefits					246		$ 245
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Policyholder Benefits		1
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Revenue		1
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Liabilities		6
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Total Liabilities	$ 80	80
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Assets and Liabilities, Net		0
Goodwill, Purchase Accounting Adjustments		0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Assets Liabilities and Goodwill, Net		$ 0
Business Acquisition, Goodwill, Expected Tax Deductible Amount					$ 610