Summary of Significant Accounting Policies - Contract Revenue Adjustments on Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets:
Accounts receivable, net of allowances	$ 648.3	$ 850.1	$ 852.0
Prepaid expenses and other current assets	253.4	331.4	299.9
Other long-term assets	373.7	394.4	415.5
Total assets	9,100.3	9,842.3	9,833.8
Liabilities:
Deferred revenue	814.1	804.9	1,030.3
Other accrued liabilities	214.8	338.1	304.3
Long-term deferred revenue	351.2	384.4	509.0
Total liabilities	4,449.4	4,836.7	5,152.9
Stockholders' Equity:
Accumulated deficit	(3,023.0)	(3,031.1)	(3,355.8)
As reported/Without Adoption of Topic 606
Assets:
Accounts receivable, net of allowances	635.3		852.0
Prepaid expenses and other current assets	232.6		299.9
Other long-term assets	371.8		415.5
Total assets	9,064.6		9,833.8
Liabilities:
Deferred revenue	1,044.7		1,030.3
Other accrued liabilities	161.2		304.3
Long-term deferred revenue	445.5		509.0
Total liabilities	4,720.7		5,152.9
Stockholders' Equity:
Accumulated deficit	(3,330.2)		$ (3,355.8)
Adjustments/ Topic 606 Impact | Accounting Standards Update 2014-09
Assets:
Accounts receivable, net of allowances	13.0	(1.9)
Prepaid expenses and other current assets	20.8	31.5
Other long-term assets	1.9	(21.1)
Total assets	35.7	8.5
Liabilities:
Deferred revenue	(230.6)	(225.4)
Other accrued liabilities	53.6	33.8
Long-term deferred revenue	(94.3)	(124.6)
Total liabilities	(271.3)	(316.2)
Stockholders' Equity:
Accumulated deficit	$ 307.2	$ 324.7