Summary of Significant Accounting Policies - Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property and Equipment
Depreciation expense	$ 76,500	$ 75,300	$ 73,400
Property and equipment, gross	753,747	709,853
Accumulated depreciation	(595,109)	(537,329)
Property and equipment, net	158,638	172,524
Building improvements
Property and Equipment
Property and equipment, gross	$ 17,974	16,817
Building improvements | Minimum
Property and Equipment
Estimated useful lives of assets	2 years
Building improvements | Maximum
Property and Equipment
Estimated useful lives of assets	10 years
Equipment
Property and Equipment
Property and equipment, gross	$ 204,632	204,743
Equipment | Minimum
Property and Equipment
Estimated useful lives of assets	3 years
Equipment | Maximum
Property and Equipment
Estimated useful lives of assets	15 years
Capital leases - property
Property and Equipment
Property and equipment, gross	$ 26,945	26,945
Capital leases - equipment
Property and Equipment
Property and equipment, gross	18,183	14,729
Capitalized internal-use software
Property and Equipment
Depreciation expense	49,500	47,600	$ 45,600
Property and equipment, gross	486,013	446,619
Property and equipment, net	$ 79,600	$ 88,200
Capitalized internal-use software | Minimum
Property and Equipment
Estimated useful lives of assets	3 years
Capitalized internal-use software | Maximum
Property and Equipment
Estimated useful lives of assets	5 years