Fair Value Measurements - Valuation of Financial Instruments that are Marked-to-Market on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets
Available-for-sale investments	$ 232,549	$ 208,603
Derivative instruments	0	0
Fair Value Measurements Recurring
Assets
Available-for-sale investments	232,549	208,603
Derivative instruments	636	7,808
Total earning assets	233,185	216,411
Liabilities
Derivative instruments	(8,636)	(25,937)
Fair Value Measurements Recurring \| 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 Recurring \| Level 2
Assets
Available-for-sale investments	232,549	208,603
Derivative instruments	636	7,808
Total earning assets	233,185	216,411
Liabilities
Derivative instruments	(8,636)	(25,937)
Fair Value Measurements Recurring \| 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 debt securities categorized neither as held-to-maturity nor trading.

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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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