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Senior Notes payable and letter of credit facilities (Details) - Unsecured Debt - 7% Senior Unsecured Notes - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Feb. 28, 2015
Debt Instrument [Line Items]      
Debt issued $ 115.0   $ 115.0
Interest rate 7.00%   7.00%
Debt Issuance Costs, Net $ 0.7    
Debt instrument, fair value disclosure $ 117.8 $ 121.7