Income taxes - Allocation of Federal and State Income Taxes between Current and Deferred Portions (Details) - USD ($) $ in Thousands |
3 Months Ended | 12 Months Ended | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Dec. 31, 2020 |
Sep. 30, 2020 |
Jun. 30, 2020 |
Mar. 31, 2020 |
Dec. 31, 2019 |
Sep. 30, 2019 |
Jun. 30, 2019 |
Mar. 31, 2019 |
Dec. 31, 2020 |
Dec. 31, 2019 |
Dec. 31, 2018 |
|
| Income Tax Disclosure [Abstract] | |||||||||||
| Current | $ 44,362 | $ 27,641 | $ 19,259 | ||||||||
| Provision for deferred income taxes | (25,530) | (1,916) | 6,359 | ||||||||
| Total | $ 13,337 | $ (2,040) | $ 7,455 | $ 80 | $ 5,718 | $ 7,718 | $ 6,314 | $ 5,975 | $ 18,832 | $ 25,725 | $ 25,618 |