Income taxes	9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income taxes	Income taxes The Company’s and the Utilities’ effective tax rates were 19% and 19%, respectively, for the nine months ended September 30, 2018. These rates differed from statutory rates, due to state income taxes and the amortization of excess deferred income taxes related to the provision in the Tax Act that lowered the federal income tax rate from 35% to 21%. In addition, certain tax return adjustments, most notably an increased pension deduction made in conjunction with the filing of the Company’s 2017 tax returns, resulted in a net income tax benefit of $5.3 million, that lowered the effective tax rate due to the additional tax benefits realized that were associated with the rate differential. The lower tax rate was partially offset by other Tax Act changes, including the non-deductibility of excess executive compensation and various fringe benefit costs. The Company’s and the Utilities’ effective tax rate were 35% and 36%, respectively, for the nine months ended September 30, 2017. Staff Accounting Bulletin No. 118 (SAB No. 118). On December 22, 2017, the SEC staff issued SAB No. 118 to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Act. In 2017, the Company calculated its best estimate of the provision for income tax expense, in accordance with its understanding of the law and available guidance. In the third quarter of 2018, adjustments, largely relating to Treasury’s depreciation regulation guidance issued in 2018 were made to the provisional tax impacts. The adjustments were due primarily to the application of 50% bonus depreciation to 2017 fourth quarter plant additions, resulting in additional regulatory liabilities totaling $11.3 million. The Company will continue to monitor the provisional impacts and update when and if additional information is received as a result of changes in the Company’s and Utilities’ interpretations and assumptions, the issuance of Internal Revenue Service and Joint Committee on Taxation guidance, and actions the Company and Utilities may take as a result of the Tax Act. The provisional tax impacts will be finalized by the end of 2018.

Income taxes

The Company’s and the Utilities’ effective tax rates were 19% and 19%, respectively, for the nine months ended September 30, 2018. These rates differed from statutory rates, due to state income taxes and the amortization of excess deferred income taxes related to the provision in the Tax Act that lowered the federal income tax rate from 35% to 21%. In addition, certain tax return adjustments, most notably an increased pension deduction made in conjunction with the filing of the Company’s 2017 tax returns, resulted in a net income tax benefit of $5.3 million, that lowered the effective tax rate due to the additional tax benefits realized that were associated with the rate differential. The lower tax rate was partially offset by other Tax Act changes, including the non-deductibility of excess executive compensation and various fringe benefit costs. The Company’s and the Utilities’ effective tax rate were 35% and 36%, respectively, for the nine months ended September 30, 2017.

Staff Accounting Bulletin No. 118 (SAB No. 118). On December 22, 2017, the SEC staff issued SAB No. 118 to address the application of GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects of the Tax Act. In 2017, the Company calculated its best estimate of the provision for income tax expense, in accordance with its understanding of the law and available guidance. In the third quarter of 2018, adjustments, largely relating to Treasury’s depreciation regulation guidance issued in 2018 were made to the provisional tax impacts. The adjustments were due primarily to the application of 50% bonus depreciation to 2017 fourth quarter plant additions, resulting in additional regulatory liabilities totaling $11.3 million. The Company will continue to monitor the provisional impacts and update when and if additional information is received as a result of changes in the Company’s and Utilities’ interpretations and assumptions, the issuance of Internal Revenue Service and Joint Committee on Taxation guidance, and actions the Company and Utilities may take as a result of the Tax Act. The provisional tax impacts will be finalized by the end of 2018.