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Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($) | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017 | Mar. 31, 2017 | Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |
Fair value assets transfers from level 1 to level 2 | $ 0 | $ 0 |  |
Fair value assets transfers from level 2 to level 1 | 0 | 0 |  |
Fair value liabilities transfers from level 1 to level 2 | 0 | 0 |  |
Fair value liabilities transfers from level 2 to level 1 | 0 | 0 |  |
Fair value assets transfers Into level 3 | 0 | 0 |  |
Fair value assets transfers out of level 3 | 0 | 0 |  |
Fair value liabilities transfers Into level 3 | 0 | 0 |  |
Fair value liabilities transfers out of level 3 | 0 | 0 |  |
Level 3 [Member] |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |
Present value of expected payments related to contingent consideration | 30,123,000 | 27,340,000 | $ 31,460,000 | $ 48,967,000
Contingent Consideration [Member] | Level 3 [Member] |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |
Present value of expected payments related to contingent consideration | 23,087,000 | 23,711,000 | $ 28,998,000 | $ 38,713,000
Contingent Consideration [Member] | Level 3 [Member] | Accounts Payable, Accrued and Other Liabilities [Member] |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |
Present value of expected payments related to contingent consideration | $ 26,700,000 | $ 23,700,000 |  |
Maximum [Member] |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |
Derivative instruments outstanding period | 60 days |  |  |

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- Definition Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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