Property and Equipment, Net (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Property and Equipment [Line Items]
Capitalized internal-use software, gross	$ 113,800		$ 109,900
Capitalized internal-use software, net	25,000		$ 23,500
Depreciation and amortization	49,940	$ 42,904
Property and Equipment [Member]
Property and Equipment [Line Items]
Depreciation and amortization	$ 45,100	$ 38,800