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Fair value measurements - Additional Information (Details) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2019 USD ($) instrument | Jun. 30, 2018 USD ($) | Jun. 30, 2019 USD ($) instrument | Jun. 30, 2018 USD ($)
Measurement Input, Credit Spread |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Weighted average discount rate | 0.0397 |  | 0.0397 |
Mortgage revenue bonds |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Beginning balance | $ 27,970 | $ 15,427 | $ 23,636 | $ 15,427
Principal payments received | 0 | 0 | 0 | 0
Purchases | 196 | 0 | 4,530 | 0
Unrealized gain (loss) included in other comprehensive income | 0 | 0 | 0 | 0
Ending balance | $ 28,166 | $ 15,427 | $ 28,166 | $ 15,427
Mortgage revenue bonds | Bank |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |
Number of financial instruments held | instrument | 2 |  | 2 |

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- Definition Value of input used to measure debt instrument, including, but not limited to, convertible and non-convertible debt. + 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 (bbb)(2) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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