Business Acquisitions and Divestitures (Details) (USD $)	12 Months Ended			3 Months Ended	9 Months Ended	0 Months Ended			3 Months Ended		0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2014	Sep. 30, 2014	Feb. 18, 2014	Nov. 08, 2013	Nov. 30, 2012	Dec. 31, 2014	Sep. 30, 2013	Dec. 04, 2012	Sep. 13, 2012	Mar. 06, 2012	Feb. 07, 2012
Business Acquisition [Line Items]
Acquisition related costs	$ 4,200,000	$ 3,100,000	$ 5,800,000
Allocation of the purchase consideration:
Goodwill	1,051,294,000	757,368,000	723,701,000						1,051,294,000
ADS Divesiture [Member]
Intangible assets acquired
Convertible note receivable converted to preferred stock amount				12,500,000
Proceeds from conversion of note receivable					12,500,000
ADS Divesiture [Member] | Level 3
Business Acquisition [Line Items]
Face value of convertible note receivable received for divestiture of a business	25,000,000.0								25,000,000.0
Prolexic
Business Acquisition [Line Items]
Total purchase consideration						392,104,000
Measurement period adjustment, increase (decrease) in goodwill						2,200,000
Goodwill, expected tax deductible amount						62,400,000
Allocation of the purchase consideration:
Cash						33,072,000
Accounts receivable						11,208,000
Property and equipment						12,225,000
Identifiable intangible assets						87,040,000
Goodwill						293,926,000
Deferred tax assets						16,340,000
Other current and long-term assets						5,664,000
Total assets acquired						459,475,000
Other current liabilities						(5,940,000)
Current deferred revenue						(5,812,000)
Deferred tax liabilities						(36,203,000)
Other liabilities assumed						(19,416,000)
Total liabilities assumed						(67,371,000)
Net assets acquired						392,104,000
Intangible assets acquired
Gross carrying amount						87,040,000
Weighted average useful life (in years)						9 years 2 months 12 days
Prolexic | Completed technologies
Intangible assets acquired
Gross carrying amount						26,800,000
Weighted average useful life (in years)						6 years 11 months
Prolexic | Customer relationships
Intangible assets acquired
Gross carrying amount						58,500,000
Weighted average useful life (in years)						10 years 5 months
Prolexic | Non-compete agreements
Intangible assets acquired
Gross carrying amount						940,000
Weighted average useful life (in years)						3 years
Prolexic | Trademarks and trade names
Intangible assets acquired
Gross carrying amount						800,000
Weighted average useful life (in years)						4 years 11 months
Velocius
Business Acquisition [Line Items]
Total purchase consideration							4,300,000
Contingent consideration liability	900,000						2,600,000		900,000
Maximum potential payout of the contingent consideration							3,000,000
Goodwill, expected tax deductible amount							300,000
Allocation of the purchase consideration:
Identifiable intangible assets							2,500,000
Goodwill							5,400,000
Intangible assets acquired
Weighted average useful life (in years)							7 years 10 months 24 days
Strategic Network
Business Acquisition [Line Items]
Total purchase consideration								55,000,000.0
Cost of acquired entity, cash paid									27,500,000	27,500,000
Goodwill, expected tax deductible amount								30,200,000
Allocation of the purchase consideration:
Identifiable intangible assets								16,100,000
Goodwill								30,200,000
Intangible assets acquired
Weighted average useful life (in years)								9 years 9 months 18 days
Verivue
Business Acquisition [Line Items]
Total purchase consideration											30,900,000
Measurement period adjustment, increase (decrease) in goodwill											(5,800,000)
Contingent consideration liability											1,200,000
Goodwill, expected tax deductible amount											3,000,000
Allocation of the purchase consideration:
Identifiable intangible assets											7,500,000
Goodwill											14,900,000
Intangible assets acquired
Weighted average useful life (in years)											6 years 4 months 24 days
FastSoft
Business Acquisition [Line Items]
Total purchase consideration												14,400,000
Measurement period adjustment, increase (decrease) in goodwill												(1,800,000)
Goodwill, expected tax deductible amount												1,700,000
Allocation of the purchase consideration:
Identifiable intangible assets												3,700,000
Goodwill												7,100,000
Intangible assets acquired
Weighted average useful life (in years)												9 years
Cotendo
Business Acquisition [Line Items]
Total purchase consideration													278,877,000
Measurement period adjustment, increase (decrease) in goodwill													(7,800,000)
Goodwill, expected tax deductible amount													44,400,000
Allocation of the purchase consideration:
Current assets, including cash and cash equivalents of $6,405													6,751,000
Cash													6,405,000
Accounts receivable													2,920,000
Property and equipment													5,812,000
Indemnification assets													6,200,000
Long-term assets													75,000
Identifiable intangible assets													43,800,000
Goodwill													233,828,000
Deferred tax liabilities													(15,376,000)
Other liabilities assumed													(5,133,000)
Net assets acquired													278,877,000
Intangible assets acquired
Gross carrying amount													43,800,000
Weighted average useful life (in years)													7 years 1 month 6 days
Cotendo | Completed technologies
Intangible assets acquired
Gross carrying amount													24,100,000
Weighted average useful life (in years)													5 years 9 months 18 days
Cotendo | Customer relationships
Intangible assets acquired
Gross carrying amount													13,400,000
Weighted average useful life (in years)													8 years 9 months 18 days
Cotendo | Non-compete agreements
Intangible assets acquired
Gross carrying amount													3,900,000
Weighted average useful life (in years)													6 years
Cotendo | Trademarks and trade names
Intangible assets acquired
Gross carrying amount													2,400,000
Weighted average useful life (in years)													9 years 9 months 18 days
Blaze
Business Acquisition [Line Items]
Total purchase consideration														19,300,000
Goodwill, expected tax deductible amount														13,500,000
Allocation of the purchase consideration:
Identifiable intangible assets														5,100,000
Goodwill														$ 15,100,000
Intangible assets acquired
Weighted average useful life (in years)														5 years 3 months 18 days