Investments in debt and equity securities - Gross Unrealized Gains and Losses (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Gains	$ 17,310	$ 18,179
Losses	5,241	3,714
Municipal
Schedule of Available-for-sale Securities [Line Items]
Gains	723	1,720
Losses	575	12
Corporate
Schedule of Available-for-sale Securities [Line Items]
Gains	6,871	7,700
Losses	2,189	2,109
Foreign
Schedule of Available-for-sale Securities [Line Items]
Gains	2,912	3,789
Losses	1,620	482
U.S. Treasury Bonds
Schedule of Available-for-sale Securities [Line Items]
Gains	4	128
Losses	186	25
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gains	6,800	4,842
Losses	$ 671	$ 1,086

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=73527792&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=73527792&loc=d3e27161-111563

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