Property and Equipment, Net (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Property and Equipment [Line Items]
Capitalized internal-use software, gross	$ 117,200		$ 117,200		$ 109,900
Capitalized internal-use software, net	24,100		24,100		$ 23,500
Depreciation and amortization	54,863	$ 50,101	156,478	$ 138,384
Property and Equipment [Member]
Property and Equipment [Line Items]
Depreciation and amortization	$ 49,700	$ 44,100	$ 141,000	$ 123,400