Reportable Segments - Revenue and Total Assets by Geographic Area (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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			2.20%		2.20%
Non-United States | Maximum
Revenues and total assets by geographic area
% of Revenue			1.40%