Income Taxes - Effective Income Tax Rate Reconciliation (Details)	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.S. statutory rate	21.00%	35.00%	35.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	3.70%	3.10%
Reversal of excess deferred income taxes	(4.60%)
State legislative change	(1.00%)
Other, net	(0.60%)	(0.20%)
Effective tax rate	18.50%	37.90%