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

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

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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.

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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.

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Investments in debt and equity securities - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) investment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Foreign debt securities	$ 671,441,000
Number of investments in an unrealized loss position \| investment	205
Number of investments in an unrealized loss positions for more than 12 months \| investment	36
Recognized other than temporary impairment	$ 0	$ 0	$ 2,700,000
Foreign
Schedule of Available-for-sale Securities [Line Items]
Foreign debt securities	228,237,000	167,027,000
Foreign \| Canada
Schedule of Available-for-sale Securities [Line Items]
Foreign debt securities	198,700,000	139,600,000
Foreign \| United Kingdom
Schedule of Available-for-sale Securities [Line Items]
Foreign debt securities	$ 24,500,000	$ 22,600,000