Investments in debt and equity securities - Gross Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Gains	$ 4,156	$ 9,958
Losses	12,176	4,163
Municipal
Debt Securities, Available-for-sale [Line Items]
Gains	463	1,263
Losses	469	175
Corporate
Debt Securities, Available-for-sale [Line Items]
Gains	2,372	6,953
Losses	7,156	497
Foreign
Debt Securities, Available-for-sale [Line Items]
Gains	1,320	1,742
Losses	4,128	3,255
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Gains	1	0
Losses	$ 423	$ 236

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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