Investments in debt and equity securities - Additional Information (Details) $ in Thousands	Dec. 31, 2018 USD ($) investment	Dec. 31, 2017 USD ($)
Investments, Debt and Equity Securities [Abstract]
Net unrealized investment gains on equity securities held	$ 2,900	$ 5,800
Debt Securities, Available-for-sale [Line Items]
Foreign debt securities	$ 602,020	671,441
Number of investments in an unrealized loss position \| investment	307
Number of investments in an unrealized loss positions for more than 12 months \| investment	145
Foreign
Debt Securities, Available-for-sale [Line Items]
Foreign debt securities	$ 198,938	228,237
Foreign \| Canada
Debt Securities, Available-for-sale [Line Items]
Foreign debt securities	171,200	198,700
Foreign \| United Kingdom
Debt Securities, Available-for-sale [Line Items]
Foreign debt securities	$ 23,000	$ 24,500

Equity Securities, Accumulated Unrealized Gain (Loss)

No definition available.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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- Definition

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

No definition available.

+ Details

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- References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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