Property and Equipment, Net (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Property and Equipment [Line Items]
Capitalized internal-use software, gross	$ 122,500		$ 122,000
Capitalized internal-use software, net	24,800		$ 26,500
Depreciation and amortization	59,031	$ 49,940
Property and Equipment [Member]
Property and Equipment [Line Items]
Depreciation and amortization	$ 54,200	$ 45,100