Fair value measurements - Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets:
Equity securities at fair value	$ 22,326	$ 25,171	$ 23,586
Fair Value, Measurements, Recurring
Assets:
Equity securities at fair value	22,326	25,171	23,586
Cash, cash equivalents, and restricted cash	16,902	9,924	10,932
Total assets	263,214	157,315	136,431
Liabilities:
Long-term notes payable		20,000	18,095
Total liabilities		20,000	18,095
Fair Value, Measurements, Recurring \| U.S. Governments
Assets:
Fair Value Assets	13,539	15,269	13,285
Fair Value, Measurements, Recurring \| States, territories, and possession
Assets:
Fair Value Assets	2,469	1,221	3,197
Fair Value, Measurements, Recurring \| Political subdivisions
Assets:
Fair Value Assets	1,955	815	4,067
Fair Value, Measurements, Recurring \| Special revenue excluding mortgage/asset-backed securities
Assets:
Fair Value Assets	18,373	12,453	23,914
Fair Value, Measurements, Recurring \| Industrial and miscellaneous
Assets:
Fair Value Assets	128,855	65,126	44,531
Fair Value, Measurements, Recurring \| Mortgage/asset-backed securities
Assets:
Fair Value Assets	58,795	27,336	12,919
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 1
Assets:
Equity securities at fair value	22,326	25,171	23,586
Cash, cash equivalents, and restricted cash	16,902	9,924	10,932
Total assets	39,228	35,095	34,518
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2
Assets:
Total assets	223,986	122,220	101,913
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| U.S. Governments
Assets:
Fair Value Assets	13,539	15,269	13,285
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| States, territories, and possession
Assets:
Fair Value Assets	2,469	1,221	3,197
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Political subdivisions
Assets:
Fair Value Assets	1,955	815	4,067
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Special revenue excluding mortgage/asset-backed securities
Assets:
Fair Value Assets	18,373	12,453	23,914
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Industrial and miscellaneous
Assets:
Fair Value Assets	128,855	65,126	44,531
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 2 \| Mortgage/asset-backed securities
Assets:
Fair Value Assets	$ 58,795	27,336	12,919
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3
Liabilities:
Long-term notes payable		20,000	18,095
Total liabilities		$ 20,000	$ 18,095

Fair value portion of cash, cash equivalents and restricted cash.

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

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- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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Fair value of financial and nonfinancial obligations.

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