Reportable Segments - Revenue and Total Assets by Geographic Area (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenues and total assets by geographic area
% of Revenue	100.00%	100.00%	100.00%	100.00%
Non-United States
Revenues and total assets by geographic area
% of total assets			3.00%		3.00%
Non-United States | Maximum
Revenues and total assets by geographic area
% of Revenue			2.50%