Investments in debt and equity securities - Additional Information (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2015 USD ($) investment	Dec. 31, 2014 USD ($)
Unrealized Losses On Investments [Line Items]
Foreign debt securities	$ 548,596
Number of investments in an unrealized loss position \| investment	155
Number of investments in an unrealized loss positions for more than 12 months \| investment	30
Investment and Other (Losses) Gains
Unrealized Losses On Investments [Line Items]
Recognized other than temporary impairment	$ 2,700
Foreign
Unrealized Losses On Investments [Line Items]
Foreign debt securities	153,221	$ 166,685
Foreign \| Canada
Unrealized Losses On Investments [Line Items]
Foreign debt securities	125,200	135,800
Foreign \| United Kingdom
Unrealized Losses On Investments [Line Items]
Foreign debt securities	$ 23,100	$ 27,300

Unrealized Losses On Investments [Line Items]

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

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Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

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- Definition

The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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