Income Taxes	6 Months Ended
	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Income Tax Disclosure [Text Block]	Income taxesThe 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 16.7% and 17.4% for the three and six months ended June 30, 2019, respectively. The Company's effective tax rates were 18.9% and 18.2% for the three and six months ended June 30, 2018, respectively. The decrease in the effective tax rates in the second quarters of 2019 and 2018 and first half of 2019 and 2018 compared to the U.S. statutory tax rate was primarily due to stock options exercised and tax-exempt interest income on certain tax-advantaged investments.