Reportable Segments - Revenue and Total Assets by Geographic Area (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues and total assets by geographic area
% of Revenue	100.00%	100.00%	100.00%
Non-United States
Revenues and total assets by geographic area
% of Revenue	1.00%
Canada
Revenues and total assets by geographic area
% of total assets	1.00%	1.00%