Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Income tax expense	$ 10,957	$ 18,189	$ 13,498	$ 29,046
Income tax expense decrease amount	7,200		15,500
Increase in tax benefits	900		900
Ratepayer net refund						$ 108,000
Unrecognized tax benefits	12,000		12,000			$ 10,200
Tax benefits that, if recognized, would affect the effective tax rate	$ 2,300		$ 2,300
Scenario, Forecast | Minimum
Income Taxes [Line Items]
Effective tax rate estimate					22.00%
Scenario, Forecast | Maximum
Income Taxes [Line Items]
Effective tax rate estimate					25.00%