Summary of Business and Significant Accounting Policies (Narrative) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) segment	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of reportable segments | segment	1
Increase in retained earnings	$ 103,764		$ 72,741
ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings		$ 6,900
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and equipment, useful life	3 years
Definite-lived intangible asset, useful life	1 year
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and equipment, useful life	5 years
Definite-lived intangible asset, useful life	10 years
Software and software development costs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and equipment, useful life	4 years