Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Income tax expense	$ 3,527	$ 10,852	$ 2,540	$ 10,857
Income tax expense decrease amount	7,300		8,300
Ratepayer net refund						$ 108,000
Unrecognized tax benefits	11,500		11,500			10,200
Tax benefits that, if recognized, would affect the effective tax rate	$ 2,200		$ 2,200			$ 2,300
Scenario, Forecast | Minimum
Income Taxes [Line Items]
Effective tax rate estimate					23.00%
Scenario, Forecast | Maximum
Income Taxes [Line Items]
Effective tax rate estimate					25.00%