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Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($) | 6 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2019 | Dec. 31, 2018 | Mar. 31, 2019 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017
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 |  |  |  |
Present value of expected payments related to contingent consideration | 68,401,000 | 49,230,000 | $ 46,143,000 | $ 36,160,000 | $ 34,489,000 | $ 26,757,000
Other Assets [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |  |
Alternative investment | 57,500,000 | 53,500,000 |  |  |  |
Other Assets [Member] | Fair Value, Measurements, Nonrecurring [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |  |
Alternative investment | 57,500,000 | 57,500,000 |  |  |  |
Contingent Consideration [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 | 38,712,000 | 32,552,000 | $ 31,357,000 | $ 21,574,000 | $ 23,088,000 | $ 23,710,000
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 | 34,200,000 | 32,600,000 |  |  |  |
Undiscounted value of payments, assuming that all contingencies are met | $ 40,200,000 | $ 39,600,000 |  |  |  |

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- Definition Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 35 -Paragraph 54B -URI http://asc.fasb.org/extlink&oid=99392525&loc=SL7495116-110257 + Details
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- Definition For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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