Property and Equipment, Net (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property and Equipment [Line Items]
Capitalized internal-use software, gross	$ 109,900	$ 107,800
Capitalized internal-use software, net	23,500	30,900
Depreciation and amortization	188,049	164,915	$ 169,662
Property and Equipment [Member]
Property and Equipment [Line Items]
Depreciation and amortization	$ 167,200	$ 143,600	$ 141,300