Reportable Segments (Geographical) (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Total Revenue | United States
Segment Reporting Information [Line Items]
Concentration risk (percent)	63.00%	65.00%
Total Revenue | United Kingdom
Segment Reporting Information [Line Items]
Concentration risk (percent)	10.00%	12.00%
Total Revenue | Australia
Segment Reporting Information [Line Items]
Concentration risk (percent)	9.00%	8.00%
Total Revenue | Germany
Segment Reporting Information [Line Items]
Concentration risk (percent)	6.00%	6.00%
Total Revenue | Poland
Segment Reporting Information [Line Items]
Concentration risk (percent)	7.00%	5.00%
Total Revenue | Russia
Segment Reporting Information [Line Items]
Concentration risk (percent)	3.00%	3.00%
Total Revenue | France
Segment Reporting Information [Line Items]
Concentration risk (percent)	2.00%	1.00%
Total Assets | United States
Segment Reporting Information [Line Items]
Concentration risk (percent)	58.00%		59.00%
Total Assets | United Kingdom
Segment Reporting Information [Line Items]
Concentration risk (percent)	12.00%		11.00%
Total Assets | Australia
Segment Reporting Information [Line Items]
Concentration risk (percent)	9.00%		9.00%
Total Assets | Germany
Segment Reporting Information [Line Items]
Concentration risk (percent)	7.00%		7.00%
Total Assets | Poland
Segment Reporting Information [Line Items]
Concentration risk (percent)	8.00%		8.00%
Total Assets | Russia
Segment Reporting Information [Line Items]
Concentration risk (percent)	2.00%		2.00%
Total Assets | France
Segment Reporting Information [Line Items]
Concentration risk (percent)	4.00%		4.00%