Property and Equipment - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Total capital leased assets, gross	$ 100,900	$ 101,300
Capital leases accumulated depreciation	43,400	37,900
Total capitalized internally developed software, gross	789,437	698,362
Capitalized internally developed software accumulated depreciation	432,123	387,798
Depreciation expense	61,400	60,800	$ 60,000
Internal Use Software
Property, Plant and Equipment [Line Items]
Total capitalized internally developed software, gross	21,100	17,800
Capitalized internally developed software accumulated depreciation	$ 13,000	$ 9,200