Business Acquisitions - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended			4 Months Ended	9 Months Ended
	Sep. 30, 2019	May 23, 2019	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Business Acquisition [Line Items]
Goodwill	$ 1,913		$ 1,913			$ 1,913	$ 1,913		$ 1,290
Revenues			5,347		$ 4,842		15,379	$ 14,322
Net Income (Loss) Available to Common Stockholders, Basic			$ 524		$ 432		1,521	$ 1,611
The Navigators Group, Inc. [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		May 23, 2019
Business Acquisition, Name of Acquired Entity		Navigators Group
Business Acquisition, Share Price		$ 70
Payments to Acquire Businesses, Gross		$ 2,121
Business Combination, Consideration Transferred, Other		23
Goodwill		623
Revenues						616
Net Income (Loss) Available to Common Stockholders, Basic						$ 140
Business Combination, Acquisition Related Costs							16
The Navigators Group, Inc. [Member] | Cash [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross		2,098
Adverse Development Cover Navigators Group [Member]
Business Acquisition [Line Items]
Reinsurance premium				$ 91
Reinsurance Retention Policy, Amount Retained				68
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims				97
Current accident year				29
Use of Retention Layer by Acquiree Prior to Acquisition		32
Policyholder Benefits and Claims Incurred, Ceded				91
Adverse Development Cover Navigators Group [Member] | Remaining [Member]
Business Acquisition [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	$ 209
Adverse Development Cover Navigators Group [Member] | After tax [Member]
Business Acquisition [Line Items]
Reinsurance premium				72
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims				$ 77
Adverse Development Cover Navigators Group [Member] | Maximum [Member]
Business Acquisition [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured							300
Recorded by Acquiree Prior to Acquisition [Member]
Business Acquisition [Line Items]
Current accident year		$ 52
Retention Layer Above Reserve for the Covered Liabilities as of the Inception Date [Member] | Adverse Development Cover Navigators Group [Member]
Business Acquisition [Line Items]
Reinsurance Retention Policy, Amount Retained							100
Retention Layer for Reserve for the Covered Liabilities as of the Inception Date [Member] | Adverse Development Cover Navigators Group [Member]
Business Acquisition [Line Items]
Reinsurance Retention Policy, Amount Retained							$ 1,816