Revenue - Contract Balances (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Contract Asset [Roll Forward]
Capitalized Contract Cost, Net, Beginning Balance	$ 32,400,000.0	$ 32,000,000.0	$ 30,600,000.0
Capitalized Contract Cost, Additions	5,500,000.0	23,200,000.0	23,600,000.0
Capitalized Contract Cost, Amortization	(5,500,000.0)	(22,800,000.0)	(22,200,000.0)
Capitalized Contract Cost, Net, Ending Balance	32,400,000.0	32,400,000.0	32,000,000.0
Fulfillment Costs [Member]
Contract Asset [Roll Forward]
Capitalized Contract Cost, Net, Beginning Balance	19,500,000.0	18,600,000.0	16,500,000.0
Capitalized Contract Cost, Additions	3,500,000.0	15,300,000.0	15,600,000.0
Capitalized Contract Cost, Amortization	(3,600,000.0)	(14,400,000.0)	(13,500,000.0)
Capitalized Contract Cost, Net, Ending Balance	19,400,000.0	19,500,000.0	18,600,000.0
Cost of Acquisition [Member]
Contract Asset [Roll Forward]
Capitalized Contract Cost, Net, Beginning Balance	12,900,000.0	13,400,000.0	14,100,000.0
Capitalized Contract Cost, Additions	2,000,000.0	7,900,000.0	8,000,000.0
Capitalized Contract Cost, Amortization	(1,900,000.0)	(8,400,000.0)	(8,700,000.0)
Capitalized Contract Cost, Net, Ending Balance	13,000,000.0	12,900,000.0	13,400,000.0
Entertainment and Communications [Member]
Contract Asset [Roll Forward]
Capitalized Contract Cost, Net, Beginning Balance	29,100,000.0	29,100,000.0	27,700,000.0
Capitalized Contract Cost, Additions	4,700,000.0	20,500,000.0	21,800,000.0
Capitalized Contract Cost, Amortization	(5,000,000.0)	(20,500,000.0)	(20,400,000.0)
Capitalized Contract Cost, Net, Ending Balance	28,800,000.0	29,100,000.0	29,100,000.0
Entertainment and Communications [Member] | Fulfillment Costs [Member]
Contract Asset [Roll Forward]
Capitalized Contract Cost, Net, Beginning Balance	17,500,000.0	17,000,000.0	15,000,000.0
Capitalized Contract Cost, Additions	3,100,000.0	13,700,000.0	14,500,000.0
Capitalized Contract Cost, Amortization	(3,300,000.0)	(13,200,000.0)	(12,500,000.0)
Capitalized Contract Cost, Net, Ending Balance	17,300,000.0	17,500,000.0	17,000,000.0
Entertainment and Communications [Member] | Cost of Acquisition [Member]
Contract Asset [Roll Forward]
Capitalized Contract Cost, Net, Beginning Balance	11,600,000.0	12,100,000.0	12,700,000.0
Capitalized Contract Cost, Additions	1,600,000.0	6,800,000.0	7,300,000.0
Capitalized Contract Cost, Amortization	(1,700,000.0)	(7,300,000.0)	(7,900,000.0)
Capitalized Contract Cost, Net, Ending Balance	11,500,000.0	11,600,000.0	12,100,000.0
IT Services and Hardware [Member]
Contract Asset [Roll Forward]
Capitalized Contract Cost, Net, Beginning Balance	3,300,000.0	2,900,000.0	2,900,000.0
Capitalized Contract Cost, Additions	800,000.0	2,700,000.0	1,800,000.0
Capitalized Contract Cost, Amortization	(500,000.0)	(2,300,000.0)	(1,800,000.0)
Capitalized Contract Cost, Net, Ending Balance	3,600,000.0	3,300,000.0	2,900,000.0
IT Services and Hardware [Member] | Fulfillment Costs [Member]
Contract Asset [Roll Forward]
Capitalized Contract Cost, Net, Beginning Balance	2,000,000.0	1,600,000.0	1,500,000.0
Capitalized Contract Cost, Additions	400,000.0	1,600,000.0	1,100,000.0
Capitalized Contract Cost, Amortization	(300,000.0)	(1,200,000.0)	(1,000,000.0)
Capitalized Contract Cost, Net, Ending Balance	2,100,000.0	2,000,000.0	1,600,000.0
IT Services and Hardware [Member] | Cost of Acquisition [Member]
Contract Asset [Roll Forward]
Capitalized Contract Cost, Net, Beginning Balance	1,300,000.0	1,300,000.0	1,400,000.0
Capitalized Contract Cost, Additions	400,000.0	1,100,000.0	700,000.0
Capitalized Contract Cost, Amortization	(200,000.0)	(1,100,000.0)	(800,000.0)
Capitalized Contract Cost, Net, Ending Balance	$ 1,500,000.0	$ 1,300,000.0	$ 1,300,000.0