Reinsurance - Unsecured Reinsurance Recoverable by Reinsurer (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Ceded Credit Risk [Line Items]
Reinsurance recoverable	$ 89.8	$ 49.0
Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	100.3	56.7
Lloyd’s Underwriter Syndicate no. 1084 CSL | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	4.4	3.0
Lloyd's Underwriter Syndicate no. 2001 AML | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	0.7	1.2
Top Reinsurers | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	97.0	51.6
Other reinsurers | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	3.3	5.1
A+ | Hannover Rueck SE | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	60.2	22.2
A+ | Swiss Reinsurance America Corporation | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	12.4	5.8
A+ | Arch Reinsurance Limited | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	2.4	2.7
A+ | Munich Reinsurance America Inc | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	0.6	1.6
A | MAPFRE Re, Compania De Reaseguros S.A. | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	14.4	6.8
A | Lloyd's Underwriter Syndicate no. 0033 HIS | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	0.8	1.6
A | Lloyd's Underwriter Syndicate no. 2357 NCL | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	0.6	1.6
A | Hiscox Insurance Company (Bermuda) Ltd | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	0.5	1.2
A++ | Tokio Marine & Nichido Fire Insurance Company Limited | Ceded Credit Risk, Unsecured
Ceded Credit Risk [Line Items]
Reinsurance recoverable	$ 0.0	$ 3.9