Significant transactions - Loss Portfolio Transfer (Details) - USD ($) $ in Millions	Jul. 07, 2020	Jan. 23, 2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash received in loss portfolio transfer agreement		$ 69.7
Cash paid in loss portfolio transfer agreement		$ 0.4
Subsequent event | Empire Insurance Company | Runoff & Other
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of stock disposed of	100.00%