Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 635,033	$ 665,646
Fair values	624,422	671,441
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	62,097	71,581
Fair values	61,703	72,669
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	346,429	351,477
Fair values	340,919	357,933
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	213,559	229,750
Fair values	209,329	228,237
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	12,948	12,838
Fair values	$ 12,471	$ 12,602

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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

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-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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+ References

No definition available.

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