Description of Business and Accounting Policies (Details) - USD ($) $ / shares in Units, shares in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of customers, exceeds 10% of total accounts receivable	1
Accounts Receivable from one customer greater than 10%, percentage	10.00%		10.00%
Percentage of Revenue by Foreign Subsidiaries	6.00%
Available-for-Sale Securities, Shares Sold		2.8
Proceeds from sale of Investment in CyrusOne	$ 0	$ 140,700,000
Gain on sale of Investment in CyrusOne	0	117,700,000
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	700,000		$ 14,900,000
Reclassification Adjustment from Accumulated Other Comprehensive Income (Loss) to Retained Earnings (Accumulated Deficit), Stranded Other Comprehensive Income Taxes Arising from Tax Reform			32,200,000
Defined Benefit Plan, Cost Reclassified, Cost of Services and Products		1,700,000
Defined Benefit Plan, Cost Reclassified, SG&A costs		1,500,000
Adjustment to revenue and cost of services and products for adoption of ASC 606		28,600,000
Reduction to cost of services and selling, general and administrative expenses		$ 300,000
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Basic Earnings Per Share		$ 0.01
Capitalized Contract Cost, Net, Incremental Assets Related to Fulfillment and Acquisition Costs			32,300,000
Capitalized Contract Cost, Net	$ 32,400,000.0		32,400,000.0	$ 32,000,000.0	$ 30,600,000.0
Decrease in deferred tax assets, impact of ASC 606			$ 7,100,000