Basis of Condensed Consolidated Financial Statements (Policies)	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In May 2017, the Financial Accounting Standards Board (“FASB”) issued updated guidance intended to reduce diversity in practice by clarifying which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. The adoption of this guidance had no impact on the Company’s condensed consolidated financial statements. In March 2017, the FASB issued updated guidance intended to improve the presentation of net periodic pension cost and net periodic postretirement benefit cost through the disaggregation of the service cost component from the other components of net benefit cost. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. The Company adopted this change in accounting principle at the beginning of 2018 and applied the change retrospectively to the prior year. As a result, other components of net benefit cost totaling $155.4 million and $171.4 million were reclassified from personnel costs to other operating expenses on the condensed consolidated statements of income for the three and nine months ended September 30, 2017, respectively. See Note 10 Employee Benefit Plans for further information on the Company’s net periodic pension costs. In January 2017, the FASB issued updated guidance to clarify the definition of a business with the objective of providing guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. The adoption of this guidance had no impact on the Company’s condensed consolidated financial statements. In November 2016, the FASB issued updated guidance intended to reduce the diversity in practice on presenting restricted cash and restricted cash equivalents in the statement of cash flows. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. The adoption of this guidance had no impact on the Company’s condensed consolidated financial statements. In October 2016, the FASB issued updated guidance intended to simplify and improve the accounting for the income tax consequences of intra-entity transfers of assets, other than inventory. The updated guidance, which eliminates the intra-entity transfers exception, requires entities to recognize the income tax consequences of intra-entity transfers of assets, other than inventory, when the transfers occur. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. The adoption of this guidance had no impact on the Company’s condensed consolidated financial statements. In August 2016, the FASB issued updated guidance intended to eliminate the diversity in practice regarding the presentation and classification of certain cash receipts and cash payments in the statement of cash flows. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. The adoption of this guidance had no impact on the Company’s condensed consolidated financial statements. In January 2016, the FASB issued updated guidance intended to enhance the reporting model for financial instruments to provide users of financial statements with more decision-useful information. In addition to making other targeted improvements to current guidance, the updated guidance also requires all equity investments, except those accounted for under the equity method of accounting or those that result in consolidation of the investee, to be measured at fair value with changes in the fair value recognized through net income. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2017. The Company adopted this guidance at the beginning of 2018 and recognized cumulative net unrealized gains, net of taxes, of $40.6 million related to its investments in equity securities, previously classified as available-for-sale, through a cumulative-effect adjustment to retained earnings. Changes in the fair values of these investments are reflected in net realized investment gains/losses on the Company’s condensed consolidated statements of income. See Note 4 Debt and Equity Securities for further discussion of the Company’s investments in equity securities. In May 2014, the FASB issued updated guidance for recognizing revenue from contracts with customers to provide a single, comprehensive revenue recognition model for all contracts with customers to improve comparability within and across industries, and across capital markets. The new revenue standard contains principles that an entity will apply to determine the measurement of revenue and the timing of recognition. The underlying principle is that an entity will recognize revenue to depict the transfer of goods or services to customers at an amount that the entity expects to be entitled to in exchange for those goods or services. Revenue from insurance contracts is not within the scope of this guidance. In August 2015, the FASB issued updated guidance which defers the effective date of this guidance by one year. In 2016, the FASB issued additional updates to the new guidance primarily to clarify, among other things, the implementation guidance related to principal versus agent considerations, identifying performance obligations, accounting for licenses of intellectual property, and to provide narrow-scope improvements and additional practical expedients. In February 2017, the FASB issued an additional update to the new guidance to clarify the scope of derecognition guidance for nonfinancial assets and to provide guidance for partial sales of nonfinancial assets. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2017. The Company elected to adopt the new guidance under the modified retrospective approach, which, except for the disclosure requirements, did not have a material impact on its condensed consolidated financial statements. See Note 2 Adoption of Revenue Guidance for further information about the Company’s revenues within the scope of the new guidance.
Pending Accounting Pronouncements	Pending Accounting Pronouncements In August 2018, the FASB issued updated guidance that is intended to reduce potential diversity in practice in accounting for the costs of implementing cloud computing arrangements (i.e., hosting arrangements) that are service contracts. The updated guidance aligns the requirements for capitalizing implementation costs for these arrangements with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software and hosting arrangements that include an internal-use software license. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted. The Company is currently assessing the impact of this guidance on its condensed consolidated financial statements. In August 2018, the FASB issued updated guidance as part of its disclosure framework project intended to improve the effectiveness of disclosures in the notes to the financial statements. The updated guidance eliminates, adds and modifies certain disclosure requirements related to fair value measurements. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted. Except for the disclosure requirements, the Company does not expect the adoption of this guidance to have a material impact on its condensed consolidated financial statements. In January 2017, the FASB issued updated guidance intended to simplify how an entity tests goodwill for impairment by eliminating Step 2 from the goodwill impairment test. Under the updated guidance, an entity will perform its goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount and will recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit’s fair value, with the loss recognized limited to the total amount of goodwill allocated to that reporting unit. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted. The Company does not expect the adoption of this guidance to have a material impact on its condensed consolidated financial statements. In June 2016, the FASB issued updated guidance intended to provide financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The updated guidance replaces the current incurred loss impairment methodology with a methodology that reflects expected credit losses and requires the consideration of a broader range of reasonable and supportable information to inform credit loss estimates. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted. The Company is currently assessing the impact of this guidance on its condensed consolidated financial statements. In February 2016, the FASB issued updated guidance that requires the rights and obligations associated with leasing arrangements be reflected on the balance sheet in order to increase transparency and comparability among organizations. Under the updated guidance, lessees will be required to recognize a right-of-use asset and a liability to make lease payments and disclose key information about leasing arrangements. The updated guidance is required to be adopted using a modified retrospective transition approach. In July 2018, the FASB issued additional updates to the new guidance which allows for the initial application of the guidance at the adoption date and for the recognition of a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2018, with early adoption permitted. While the Company is currently evaluating the impact the new guidance will have on its condensed consolidated financial statements, the Company expects the adoption of the new guidance will result in a material increase in the assets and liabilities on its condensed consolidated balance sheets and will likely have an insignificant impact on its condensed consolidated statements of income and statements of cash flows.