Restructuring - Summary of Accruals of Restructuring Programs (Details) - USD ($) $ in Thousands	3 Months Ended						12 Months Ended			24 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Restructuring Reserve [Roll Forward]
Restructuring Charges	$ 3,400	$ 800	$ 13,500	$ 300	$ 15,500	$ 4,700	$ 17,994	$ 20,218	$ 0
2016 and 2017 Restructuring Program
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance				12,662			12,662	0		$ 0
Restructuring Charges							16,677	15,713		38,200
Adjustments							(69)	(241)
Total Restructuring Charges							16,608	15,472
Payments							(14,530)	(2,810)
Restructuring reserve, ending balance	14,740				12,662		14,740	12,662	0	14,740
2016 and 2017 Restructuring Program | Contractual Lease Obligations
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance				10,583			10,583	0		0
Restructuring Charges							13,501	11,830
Adjustments							(69)	(241)
Total Restructuring Charges							13,432	11,589
Payments							(11,578)	(1,006)
Restructuring reserve, ending balance	12,437				10,583		12,437	10,583	0	12,437
2016 and 2017 Restructuring Program | Severance
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning balance				$ 2,079			2,079	0		0
Restructuring Charges							3,176	3,883
Adjustments							0	0
Total Restructuring Charges							3,176	3,883
Payments							(2,952)	(1,804)
Restructuring reserve, ending balance	$ 2,303				$ 2,079		$ 2,303	$ 2,079	$ 0	$ 2,303