Property and Equipment, Net (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Property and Equipment [Line Items]
Capitalized internal-use software, gross	$ 115,800		$ 115,800		$ 109,900
Capitalized internal-use software, net	25,000		25,000		$ 23,500
Depreciation and amortization	51,676	$ 45,379	101,615	$ 88,282
Property and Equipment [Member]
Property and Equipment [Line Items]
Depreciation and amortization	$ 46,300	$ 40,500	$ 91,300	$ 79,200