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Basis of Presentation - Additional Information (Detail) - shares shares in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Subsidiary or Equity Method Investee [Line Items] |  |  |  |
Shares excluded from diluted EPS calculation | 0.4 | 3.3 | 0.4 | 4.7
CNA Financial [Member] |  |  |  |
Subsidiary or Equity Method Investee [Line Items] |  |  |  |
Subsidiary ownership percentage |  |  | 89.00% |
Diamond Offshore Drilling, Inc. [Member] |  |  |  |
Subsidiary or Equity Method Investee [Line Items] |  |  |  |
Subsidiary ownership percentage |  |  | 53.00% |
Boardwalk Pipeline Partners, LP [Member] |  |  |  |
Subsidiary or Equity Method Investee [Line Items] |  |  |  |
Subsidiary ownership percentage |  |  | 51.00% |
Consolidated Container Company [Member] |  |  |  |
Subsidiary or Equity Method Investee [Line Items] |  |  |  |
Subsidiary ownership percentage |  |  | 99.00% |

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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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- Definition Percentage of subsidiary's or equity method investee's stock owned by parent immediately after all stock transactions. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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