Investments in debt and equity securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Held to maturity	$ 3,402	$ 31,630
Available for sale	35,544	38,895
Total	38,946	70,525
Held to maturity, short-term investments	3,402	31,630
Available for sale, short-term investments	1,200	1,200
Total, short-term investments	4,602	32,830
Held to maturity, long-term investments	0	0
Available for sale, long-term investments	34,344	37,695
Total, long-term investments	34,344	37,695
Certificates of deposit and other time deposits
Investment Holdings [Line Items]
Held to maturity	3,402	31,630
Available for sale, equity securities	0	0
Total	3,402	31,630
Investments in mutual funds and common stock
Investment Holdings [Line Items]
Held to maturity	0	0
Available for sale, equity securities	35,544	38,895
Total	$ 35,544	$ 38,895

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of equity securities categorized neither as held-to-maturity nor as trading.

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Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle.

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