Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Derivative instruments	$ 58,140	$ 8,285
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Fair value of derivative liabilities, Gross amounts	7,378	40,847
Fair value, measurements, recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash	639,941	985,382
Derivative instruments	58,140	8,285
Total financial assets	944,041	1,458,316
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Fair value of derivative liabilities, Gross amounts	7,378	40,847
Total financial liabilities	7,378	40,847
Fair value, measurements, recurring [Member] \| Money market and deposit accounts [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Financial assets	230,545	427,135
Fair value, measurements, recurring [Member] \| Publicly traded equity securities [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Financial assets	4,098	6,163
Fair value, measurements, recurring [Member] \| Certificates of deposit [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Financial assets	11,317	31,351
Fair value, measurements, recurring [Member] \| Level 1 [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash	639,941	985,382
Derivative instruments	0	0
Total financial assets	874,584	1,418,680
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Fair value of derivative liabilities, Gross amounts	0	0
Total financial liabilities	0	0
Fair value, measurements, recurring [Member] \| Level 1 [Member] \| Money market and deposit accounts [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Financial assets	230,545	427,135
Fair value, measurements, recurring [Member] \| Level 1 [Member] \| Publicly traded equity securities [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Financial assets	4,098	6,163
Fair value, measurements, recurring [Member] \| Level 1 [Member] \| Certificates of deposit [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Financial assets	0	0
Fair value, measurements, recurring [Member] \| Level 2 [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash	0	0
Derivative instruments	58,140	8,285
Total financial assets	69,457	39,636
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Fair value of derivative liabilities, Gross amounts	7,378	40,847
Total financial liabilities	7,378	40,847
Fair value, measurements, recurring [Member] \| Level 2 [Member] \| Money market and deposit accounts [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Financial assets	0	0
Fair value, measurements, recurring [Member] \| Level 2 [Member] \| Publicly traded equity securities [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Financial assets	0	0
Fair value, measurements, recurring [Member] \| Level 2 [Member] \| Certificates of deposit [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Financial assets	11,317	$ 31,351
ASU 2016-01 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cumulative effect on retained earnings	$ 2,100

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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- Definition

Amount of the cumulative effect on retained earnings net of related income tax effect.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 9
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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

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-Publisher FASB
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