Income Taxes	9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Tax Disclosure [Text Block]	Income taxes On December 22, 2017, the President of the United States signed into law the TCJA. The legislation significantly changed U.S. tax law by, among other things, lowering corporate income tax rates from 35% to 21% effective January 1, 2018, and modifying the manner in which property and casualty insurance loss reserves are computed for federal income tax purposes. The Company records a deduction for unpaid losses and loss adjustment expenses when computing its taxable income. Prior to the new legislation, the deduction was discounted using interest rates and loss payment patterns prescribed by the U.S. Treasury. The TCJA changed the prescribed interest rates, which are now based on corporate bond yield curves, and extended the applicable time periods for the loss payment pattern period for long-tailed lines of business. Beginning in 2018, a transition rule will spread the adjustments related to pre-effective-date losses and loss adjustment expenses over the next eight years. U.S. GAAP requires companies to recognize the effect of tax law changes in the period of enactment. The SEC staff issued Staff Accounting Bulletin No. 118 (“SAB 118”) to address the application of U.S. 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 TCJA. The TCJA did not specify the application of certain elements of the legislation and the U.S. Treasury has yet to issue interpretive guidance to specify the loss payment patterns and the corporate bond yield curve under the new law for 2018. The Company recognized provisional tax amounts of $3.5 million related to the transition adjustment for loss discounting, which was included in its components of deferred tax assets and liabilities as part of its consolidated financial statements for the year ended December 31, 2017. The ultimate impact may differ from these provisional amounts due to, among other things, additional analysis, changes in interpretations and assumptions the Company has made, additional regulatory guidance that may be issued, and actions the Company may take as a result of the TCJA. During the nine months ended September 30, 2018, there were no changes to these amounts and no measurement period adjustments were recorded. The accounting is expected to be complete when the U.S. Treasury issues further guidance or within the measurement period that ends in the fourth quarter of 2018. The Company uses the estimated annual effective tax rate method for calculating its tax provision in interim periods, which represents the Company's best estimate of the effective tax rate expected for the full year. The estimated annual effective tax rate typically differs from the U.S. statutory tax rate primarily as a result of tax-exempt investment income and any discrete items recognized during the period. The Company's effective tax rates were 15.3% and 20.1% for the three months ended September 30, 2018 and 2017, respectively. The decrease in the effective tax rate in the third quarter of 2018 compared to the third quarter of 2017 was primarily due to the impact of the TCJA, which lowered the U.S. statutory tax rate from 35% to 21% starting January 1, 2018, offset in part by a larger percentage of tax benefits in the prior-year period from stock options exercised and tax-exempt interest income relative to pre-tax income. The Company's effective tax rates were 17.1% and 30.5% for the nine months ended September 30, 2018 and 2017, respectively. The decrease in the effective tax rate for the nine months ended September 30, 2018 compared to the same period in the prior year was primarily due to the impact of the TCJA.

Income taxes

On December 22, 2017, the President of the United States signed into law the TCJA. The legislation significantly changed U.S. tax law by, among other things, lowering corporate income tax rates from 35% to 21% effective January 1, 2018, and modifying the manner in which property and casualty insurance loss reserves are computed for federal income tax purposes. The Company records a deduction for unpaid losses and loss adjustment expenses when computing its taxable income. Prior to the new legislation, the deduction was discounted using interest rates and loss payment patterns prescribed by the U.S. Treasury. The TCJA changed the prescribed interest rates, which are now based on corporate bond yield curves, and extended the applicable time periods for the loss payment pattern period for long-tailed lines of business. Beginning in 2018, a transition rule will spread the adjustments related to pre-effective-date losses and loss adjustment expenses over the next eight years.

U.S. GAAP requires companies to recognize the effect of tax law changes in the period of enactment. The SEC staff issued Staff Accounting Bulletin No. 118 (“SAB 118”) to address the application of U.S. 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 TCJA. The TCJA did not specify the application of certain elements of the legislation and the U.S. Treasury has yet to issue interpretive guidance to specify the loss payment patterns and the corporate bond yield curve under the new law for 2018. The Company recognized provisional tax amounts of $3.5 million related to the transition adjustment for loss discounting, which was included in its components of deferred tax assets and liabilities as part of its consolidated financial statements for the year ended December 31, 2017. The ultimate impact may differ from these provisional amounts due to, among other things, additional analysis, changes in interpretations and assumptions the Company has made, additional regulatory guidance that may be issued, and actions the Company may take as a result of the TCJA. During the nine months ended September 30, 2018, there were no changes to these amounts and no measurement period adjustments were recorded. The accounting is expected to be complete when the U.S. Treasury issues further guidance or within the measurement period that ends in the fourth quarter of 2018.

The Company uses the estimated annual effective tax rate method for calculating its tax provision in interim periods, which represents the Company's best estimate of the effective tax rate expected for the full year. The estimated annual effective tax rate typically differs from the U.S. statutory tax rate primarily as a result of tax-exempt investment income and any discrete items recognized during the period. The Company's effective tax rates were 15.3% and 20.1% for the three months ended September 30, 2018 and 2017, respectively. The decrease in the effective tax rate in the third quarter of 2018 compared to the third quarter of 2017 was primarily due to the impact of the TCJA, which lowered the U.S. statutory tax rate from 35% to 21% starting January 1, 2018, offset in part by a larger percentage of tax benefits in the prior-year period from stock options exercised and tax-exempt interest income relative to pre-tax income. The Company's effective tax rates were 17.1% and 30.5% for the nine months ended September 30, 2018 and 2017, respectively. The decrease in the effective tax rate for the nine months ended September 30, 2018 compared to the same period in the prior year was primarily due to the impact of the TCJA.