Basis of Presentation and Significant Accounting Policies (Details) - USD ($)					3 Months Ended	12 Months Ended
	Jan. 01, 2020	May 23, 2019	Jan. 01, 2019	Jan. 01, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect				$ (88,000,000)
Equity securities, at fair value				1,000,000,000.0		$ 1,657,000,000	$ 1,214,000,000
Operating Lease, Liability						201,000,000
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability						42,000,000
Reserve for future policy benefits						635,000,000	642,000,000
Valuation allowances and reserves						145,000,000	135,000,000
Participating dividends to policyholders						30,000,000	23,000,000	$ 35,000,000
Accumulated depreciation						1,900,000,000	1,600,000,000
Depreciation expense						$ 283,000,000	$ 232,000,000	$ 197,000,000
Minimum | Other Intangible Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Useful lives						1 year
Maximum | Other Intangible Assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Useful lives						15 years
Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Discontinued Operation, Amount of Adjustment to Prior Period Gain (Loss) on Disposal, Net of Tax					$ (193,000,000)
Continuing Operations [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect				(105,000,000)
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
Property and Casualty Insurance Products
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Participating Insurance, Percentage of Premium Income						9.00%	10.00%	10.00%
Accounting Standards Update 2016-01 [Member] | Equity securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting changes, net of tax				83,000,000
Accounting Standards Update 2016-01 [Member] | Shadow DAC [Member] | Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting changes, net of tax				$ 10,000,000
Accounting Standards Update 2014-09 [Member] | Insurance Operating Costs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income								$ 188
Accounting Standards Update 2017-12 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect on Retained Earnings, before Tax			$ 1,000,000
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Liability			160,000,000
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability			150,000,000
Incentive to Lessee			$ 10,000,000
Accounting Standards Update 2016-13 [Member] | Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 18,000,000
Retroactive Reinsurance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Paid loss and loss adjustment expenses						$ 747	$ 523
Adverse Development Cover Navigators Group [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Revenue						16,000,000	0
Deferred Revenue, Additions						$ 16,000,000	$ 0	$ 0