Investments - Debt securities by contractual maturity (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Cost or amortized cost
Due in one year or less	$ 145.6	$ 50.0
Due after one year through five years	870.4	2.8
Due after five years through ten years	69.6	0.0
Due after ten years	43.6	29.4
Cost or amortized cost	2,099.3	91.4
Fair value
Due in one year or less	145.1	50.6
Due after one year through five years	862.4	3.0
Due after five years through ten years	68.6	0.0
Due after ten years	44.4	37.7
Fair value	2,085.6	101.3
Mortgage-backed and asset-backed securities
Cost or amortized cost
Debt securities without single maturity date	967.5	9.2
Fair value
Debt securities without single maturity date	962.3	10.0
Preferred stocks
Cost or amortized cost
Debt securities without single maturity date	2.6	0.0
Cost or amortized cost	2.6
Fair value
Debt securities without single maturity date	2.8	$ 0.0
Fair value	$ 2.8