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Acquisitions - Summary of Preliminary Allocation of Purchase Price to Fair Value of Assets Acquired and Liabilities Assumed (Parenthetical) (Detail) - USD ($) $ in Thousands |  | 3 Months Ended | 6 Months Ended
--- | --- | --- | ---
Jan. 03, 2019 | Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
Business Acquisition [Line Items] |  |  |  |  |
Amortization of intangible assets |  | $ 5,099 | $ 182 | $ 11,309 | $ 368
SiriusDecisions, Inc [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Acquired intangible assets | $ 115,000 |  |  |  |
Acquired intangible assets, Useful life | 8 years 4 months 24 days |  |  |  |
Amortization of intangible assets |  | $ 4,900 |  | $ 10,900 |
SiriusDecisions, Inc [Member] | Technology Asset Class [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Acquired intangible assets | $ 13,000 |  |  |  |
Acquired intangible assets, Useful life | 3 years 2 months 12 days |  |  |  |
SiriusDecisions, Inc [Member] | Technology Asset Class [Member] | Minimum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Acquired intangible assets, Useful life | 1 year |  |  |  |
SiriusDecisions, Inc [Member] | Technology Asset Class [Member] | Maximum [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Acquired intangible assets, Useful life | 8 years |  |  |  |
SiriusDecisions, Inc [Member] | Customer Relationships [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Acquired intangible assets | $ 77,000 |  |  |  |
Acquired intangible assets, Useful life | 9 years 3 months |  |  |  |
SiriusDecisions, Inc [Member] | Trade Names [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Acquired intangible assets | $ 12,000 |  |  |  |
Acquired intangible assets, Useful life | 15 years 6 months |  |  |  |
SiriusDecisions, Inc [Member] | Backlog [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Acquired intangible assets | $ 13,000 |  |  |  |
Acquired intangible assets, Useful life | 2 years |  |  |  |

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- Definition Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275 + Details
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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 + Details
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- Definition Amount of intangible assets, excluding goodwill, acquired at the acquisition date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84161108&loc=d3e4845-128472 + Details
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