Business Acquisitions - Additional Information (Details) - USD ($)			2 Months Ended	3 Months Ended								7 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2019	May 23, 2019	Dec. 31, 2017	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 01, 2017
Business Acquisition [Line Items]
Goodwill	$ 1,913,000,000		$ 1,290,000,000	$ 1,913,000,000				$ 1,290,000,000				$ 1,913,000,000		$ 1,913,000,000	$ 1,290,000,000	$ 1,290,000,000
Change in Deferred Gain on Retroactive Reinsurance	16,000,000		0	16,000,000				0				16,000,000		16,000,000	0	0
Change in Deferred Gain on Retroactive Reinsurance, Net of Tax	13,000,000			13,000,000								13,000,000		13,000,000
Net investment income														1,951,000,000	1,780,000,000	1,603,000,000
Revenues				5,361,000,000	$ 5,347,000,000	$ 5,092,000,000	$ 4,940,000,000	4,633,000,000	$ 4,842,000,000	$ 4,789,000,000	$ 4,691,000,000			20,740,000,000	18,955,000,000	17,162,000,000
Net income (loss) available to common stockholders				543,000,000	$ 524,000,000	372,000,000	$ 625,000,000	190,000,000	432,000,000	$ 582,000,000	$ 597,000,000			2,064,000,000	1,801,000,000	$ (3,131,000,000)
Adverse Development Cover Navigators Group [Member]
Business Acquisition [Line Items]
Reinsurance premium						91,000,000						91,000,000
Reinsurance Retention Policy, Amount Retained						68,000,000
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims						97,000,000						97,000,000		84,000,000
Current accident year						29,000,000
Use of Retention Layer by Acquiree Prior to Acquisition		$ 32,000,000
Adverse development from comprehensive annual review						$ 91,000,000								107,000,000
Change in Deferred Gain on Retroactive Reinsurance	16,000,000			$ 16,000,000								16,000,000		16,000,000
Adverse Development Cover Navigators Group [Member] | Remaining [Member]
Business Acquisition [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	$ 193,000,000
Adverse Development Cover Navigators Group [Member] | After tax [Member]
Business Acquisition [Line Items]
Reinsurance premium												72,000,000
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims												77,000,000
Adverse Development Cover Navigators Group [Member] | Maximum
Business Acquisition [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured														300,000,000
Adverse Development Cover Navigators Group [Member] | Retention Layer Above Reserve for the Covered Liabilities as of the Inception Date [Member]
Business Acquisition [Line Items]
Reinsurance Retention Policy, Amount Retained														100,000,000
Adverse development from comprehensive annual review														207,000,000
Adverse Development Cover Navigators Group [Member] | Retention Layer for Reserve for the Covered Liabilities as of the Inception Date [Member]
Business Acquisition [Line Items]
Reinsurance Retention Policy, Amount Retained														1,816,000,000
Recorded by Acquiree Prior to Acquisition [Member]
Business Acquisition [Line Items]
Current accident year		$ 52,000,000
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		$ 2,121,000,000
Business Combination, Consideration Transferred, Other		23,000,000
Goodwill		623,000,000
Net investment income												67,000,000
Net Investment Income, after Tax												54,000,000
Revenues												1,000,000,000.0
Net income (loss) available to common stockholders												$ 167,000,000
Business Combination, Acquisition Related Costs														$ 17,000,000
Business Combination, Consideration Transferred		$ 2,137,000,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		10 years
The Navigators Group, Inc. [Member] | Cash
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross		$ 2,098,000,000.000
Aetna Group Insurance [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross			1,450,000,000
Business Combination, Consideration Transferred, Other			2,000,000
Goodwill								$ 723,000,000							$ 723,000,000		$ 723,000,000
Revenues			370,000,000
Net income (loss) available to common stockholders			37,000,000
Business Combination, Acquisition Related Costs			17,000,000
Business Combination, Consideration Transferred			$ 1,452,000,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Total Assets									80,000,000				$ 80,000,000
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Total Liabilities									$ 80,000,000				$ 80,000,000
Aetna Group Insurance [Member] | Marketing agreement with Aetna
Business Acquisition [Line Items]
Finite-lived Intangible Assets Acquired, Term of Agreement			3 years
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			15 years
Aetna Group Insurance [Member] | Computer Software, Intangible Asset [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			5 years
Aetna Group Insurance [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Goodwill																	$ 723,000,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			15 years
Aetna Group Insurance [Member] | Intangible Assets Arising from Insurance Contracts Acquired in Business Combination [Member]
Business Acquisition [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			12 years