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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

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