Investments in debt and equity securities - Gross Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Gains	$ 16,325	$ 17,310
Losses	4,748	5,241
Municipal
Schedule of Available-for-sale Securities [Line Items]
Gains	1,263	723
Losses	175	575
Corporate
Schedule of Available-for-sale Securities [Line Items]
Gains	6,953	6,871
Losses	497	2,189
Foreign
Schedule of Available-for-sale Securities [Line Items]
Gains	1,742	2,912
Losses	3,255	1,620
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Gains	0	4
Losses	236	186
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gains	6,367	6,800
Losses	$ 585	$ 671

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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