Basis of Presentation and Principles of Consolidation	6 Months Ended
Jun. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	1. Basis of Presentation and Principles of Consolidation The accompanying unaudited consolidated financial statements of The Hanover Insurance Group, Inc. and subsidiaries (“THG” or the “Company”) have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial information and with the requirements of Form 10-Q. Certain financial information that is provided in annual financial statements, but is not required in interim reports, has been omitted. The interim consolidated financial statements of THG include the accounts of The Hanover Insurance Company (“Hanover Insurance”) and Citizens Insurance Company of America, THG’s principal U.S.-domiciled property and casualty companies; Chaucer Holdings Limited (“Chaucer”), a specialist insurance underwriting group which operates through the Society and Corporation of Lloyd’s (“Lloyd’s”) and certain other insurance and non-insurance subsidiaries. These legal entities conduct their operations through several business segments discussed in Note 8 – “Segment Information”. Additionally, the interim consolidated financial statements include the Company’s discontinued operations, consisting primarily of the Company’s accident and health and former life insurance businesses. All intercompany accounts and transactions have been eliminated. The preparation of financial statements in conformity with U.S. GAAP requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amount of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the first quarter, the effect of the adoption of new accounting standards ASC Update No. 2016-01 and ASC Update No. 2018-02 was incorrectly reflected in the presentation of other comprehensive income. The presentation of the adoption has been revised to only be presented in the Statement of Shareholders' Equity for the six months ended June 30, 2018. The revision decreased the Company's previously reported three months ended March 31, 2018 total other comprehensive loss, net of tax of $228.0 million and comprehensive loss of $160.3 million, by $101.7 million. The six months ended June 30, 2018 total other comprehensive loss, net of tax and comprehensive loss, reflecting the correction to the prior quarter, are $175.0 million and $8.0 million, respectively. The Company evaluated the significance of this item and concluded that it was not material to its previously issued interim financial statements. The associated corrections are reflected in this Form 10-Q report for all periods presented and will be reflected in the Company's future filings. In the opinion of the Company’s management, the accompanying interim consolidated financial statements reflect all adjustments, consisting of normal recurring items, necessary for a fair presentation of the financial position and results of operations. The results of operations for the three and six months ended June 30, 2018 are not necessarily indicative of the results to be expected for the full year. These financial statements should be read in conjunction with the Company’s 2017 Annual Report on Form 10-K filed with the Securities and Exchange Commission (“SEC”) on February 27, 2018.

1. Basis of Presentation and Principles of Consolidation

The accompanying unaudited consolidated financial statements of The Hanover Insurance Group, Inc. and subsidiaries (“THG” or the “Company”) have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial information and with the requirements of Form 10-Q. Certain financial information that is provided in annual financial statements, but is not required in interim reports, has been omitted.

The interim consolidated financial statements of THG include the accounts of The Hanover Insurance Company (“Hanover Insurance”) and Citizens Insurance Company of America, THG’s principal U.S.-domiciled property and casualty companies; Chaucer Holdings Limited (“Chaucer”), a specialist insurance underwriting group which operates through the Society and Corporation of Lloyd’s (“Lloyd’s”) and certain other insurance and non-insurance subsidiaries. These legal entities conduct their operations through several business segments discussed in Note 8 – “Segment Information”. Additionally, the interim consolidated financial statements include the Company’s discontinued operations, consisting primarily of the Company’s accident and health and former life insurance businesses. All intercompany accounts and transactions have been eliminated.

The preparation of financial statements in conformity with U.S. GAAP requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amount of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the first quarter, the effect of the adoption of new accounting standards ASC Update No. 2016-01 and ASC Update No. 2018-02 was incorrectly reflected in the presentation of other comprehensive income. The presentation of the adoption has been revised to only be presented in the Statement of Shareholders' Equity for the six months ended June 30, 2018. The revision decreased the Company's previously reported three months ended March 31, 2018 total other comprehensive loss, net of tax of $228.0 million and comprehensive loss of $160.3 million, by $101.7 million. The six months ended June 30, 2018 total other comprehensive loss, net of tax and comprehensive loss, reflecting the correction to the prior quarter, are $175.0 million and $8.0 million, respectively. The Company evaluated the significance of this item and concluded that it was not material to its previously issued interim financial statements. The associated corrections are reflected in this Form 10-Q report for all periods presented and will be reflected in the Company's future filings.

In the opinion of the Company’s management, the accompanying interim consolidated financial statements reflect all adjustments, consisting of normal recurring items, necessary for a fair presentation of the financial position and results of operations. The results of operations for the three and six months ended June 30, 2018 are not necessarily indicative of the results to be expected for the full year. These financial statements should be read in conjunction with the Company’s 2017 Annual Report on Form 10-K filed with the Securities and Exchange Commission (“SEC”) on February 27, 2018.