Fair Value Measurements - Valuation of Financial Instruments that are Marked-to-Market on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Available-for-sale investments	$ 172,370	$ 244,088
Fair Value Measurements on a Recurring Basis
Assets
Available-for-sale investments	172,370	244,088
Derivative instruments	1,384	812
Total earning assets	173,754	244,900
Liabilities
Derivative instruments	(454)	(2,584)
Fair Value Measurements on a Recurring Basis \| Level 1
Assets
Available-for-sale investments	0	0
Derivative instruments	0	0
Total earning assets	0	0
Liabilities
Derivative instruments	0	0
Fair Value Measurements on a Recurring Basis \| Level 2
Assets
Available-for-sale investments	172,370	244,088
Derivative instruments	1,384	812
Total earning assets	173,754	244,900
Liabilities
Derivative instruments	(454)	(2,584)
Fair Value Measurements on a Recurring Basis \| Level 3
Assets
Available-for-sale investments	0	0
Derivative instruments	0	0
Total earning assets	0	0
Liabilities
Derivative instruments	$ 0	$ 0

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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value, after the 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 not subject to a master netting arrangement and not elected to be offset.

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