Fair value measurements - Investments categorized by the level of the fair value hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2021	Feb. 26, 2021	Dec. 31, 2020
Assets
Debt securities	$ 2,085.6		$ 101.3
Equity securities	2.8		0.0
Short-term investments	1,075.8		0.0
Other long-term investments	456.1		4.0
Total assets	4,530.5		1,162.1
Liabilities
Securities sold, not yet purchased	0.0		12.0
Liability-classified capital instruments	87.8	$ 137.6	0.0
Derivative liabilities	3.2		1.0
Total liabilities	91.0		13.0
Asset-backed securities
Assets
Debt securities	513.1		1.3
Residential mortgage-backed securities
Assets
Debt securities	301.9		8.7
Commercial mortgage-backed securities
Assets
Debt securities	147.3
Bank debt
Assets
Debt securities			0.4
Corporate debt securities
Assets
Debt securities	602.6		37.7
U.S. government and government agency
Assets
Debt securities	385.4		53.2
Liabilities
Securities sold, not yet purchased			12.0
Non-U.S. government and government agency
Assets
Debt securities	132.3
U.S. states, municipalities and political subdivision
Assets
Debt securities	0.2
Preferred stocks
Assets
Debt securities	2.8
Fixed income mutual funds
Assets
Equity securities	2.1
Common stocks
Assets
Equity securities	0.7
Fair Value, Inputs, Level 1, 2 and 3
Assets
Debt securities	2,085.6		101.3
Equity securities	2.8
Short-term investments	1,075.8
Other long-term investments	336.9		4.0
Derivative assets	0.6		1.2
Total assets	3,501.7		106.5
Fair Value, Inputs, Level 1, 2 and 3 | Asset-backed securities
Assets
Debt securities	513.1		1.3
Fair Value, Inputs, Level 1, 2 and 3 | Residential mortgage-backed securities
Assets
Debt securities	301.9		8.7
Fair Value, Inputs, Level 1, 2 and 3 | Commercial mortgage-backed securities
Assets
Debt securities	147.3
Fair Value, Inputs, Level 1, 2 and 3 | Bank debt
Assets
Debt securities			0.4
Fair Value, Inputs, Level 1, 2 and 3 | Corporate debt securities
Assets
Debt securities	602.6		37.7
Fair Value, Inputs, Level 1, 2 and 3 | U.S. government and government agency
Assets
Debt securities	385.4		53.2
Fair Value, Inputs, Level 1, 2 and 3 | Non-U.S. government and government agency
Assets
Debt securities	132.3
Fair Value, Inputs, Level 1, 2 and 3 | U.S. states, municipalities and political subdivision
Assets
Debt securities	0.2
Fair Value, Inputs, Level 1, 2 and 3 | Preferred stocks
Assets
Debt securities	2.8
Fair Value, Inputs, Level 1, 2 and 3 | Fixed income mutual funds
Assets
Equity securities	2.1
Fair Value, Inputs, Level 1, 2 and 3 | Common stocks
Assets
Equity securities	0.7
Quoted prices in active markets (Level 1)
Assets
Debt securities	378.7		0.0
Equity securities	2.8
Short-term investments	1,073.2
Other long-term investments	0.0		0.0
Derivative assets	0.2		0.0
Total assets	1,454.9		0.0
Liabilities
Securities sold, not yet purchased			0.0
Liability-classified capital instruments	0.0
Derivative liabilities	0.0		0.0
Total liabilities	0.0		0.0
Quoted prices in active markets (Level 1) | Asset-backed securities
Assets
Debt securities	0.0		0.0
Quoted prices in active markets (Level 1) | Residential mortgage-backed securities
Assets
Debt securities	0.0		0.0
Quoted prices in active markets (Level 1) | Commercial mortgage-backed securities
Assets
Debt securities	0.0
Quoted prices in active markets (Level 1) | Bank debt
Assets
Debt securities			0.0
Quoted prices in active markets (Level 1) | Corporate debt securities
Assets
Debt securities	0.0		0.0
Quoted prices in active markets (Level 1) | U.S. government and government agency
Assets
Debt securities	360.9		0.0
Liabilities
Securities sold, not yet purchased			0.0
Quoted prices in active markets (Level 1) | Non-U.S. government and government agency
Assets
Debt securities	17.8
Quoted prices in active markets (Level 1) | U.S. states, municipalities and political subdivision
Assets
Debt securities	0.0
Quoted prices in active markets (Level 1) | Preferred stocks
Assets
Debt securities	0.0
Quoted prices in active markets (Level 1) | Fixed income mutual funds
Assets
Equity securities	2.1
Quoted prices in active markets (Level 1) | Common stocks
Assets
Equity securities	0.7
Significant other observable inputs (Level 2)
Assets
Debt securities	1,704.1		101.3
Equity securities	0.0
Short-term investments	2.6
Other long-term investments	0.0		0.0
Derivative assets	0.0		0.0
Total assets	1,706.7		101.3
Liabilities
Securities sold, not yet purchased			12.0
Liability-classified capital instruments	30.6
Derivative liabilities	0.0		0.0
Total liabilities	30.6		12.0
Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Debt securities	513.1		1.3
Significant other observable inputs (Level 2) | Residential mortgage-backed securities
Assets
Debt securities	301.9		8.7
Significant other observable inputs (Level 2) | Commercial mortgage-backed securities
Assets
Debt securities	147.3
Significant other observable inputs (Level 2) | Bank debt
Assets
Debt securities			0.4
Significant other observable inputs (Level 2) | Corporate debt securities
Assets
Debt securities	602.6		37.7
Significant other observable inputs (Level 2) | U.S. government and government agency
Assets
Debt securities	24.5		53.2
Liabilities
Securities sold, not yet purchased			12.0
Significant other observable inputs (Level 2) | Non-U.S. government and government agency
Assets
Debt securities	114.5
Significant other observable inputs (Level 2) | U.S. states, municipalities and political subdivision
Assets
Debt securities	0.2
Significant other observable inputs (Level 2) | Preferred stocks
Assets
Debt securities	0.0
Significant other observable inputs (Level 2) | Fixed income mutual funds
Assets
Equity securities	0.0
Significant other observable inputs (Level 2) | Common stocks
Assets
Equity securities	0.0
Significant unobservable inputs (Level 3)
Assets
Debt securities	2.8		0.0
Equity securities	0.0
Short-term investments	0.0
Other long-term investments	336.9		4.0
Derivative assets	0.4		1.2
Total assets	340.1		5.2
Liabilities
Securities sold, not yet purchased			0.0
Liability-classified capital instruments	57.2
Derivative liabilities	3.2		1.0
Total liabilities	60.4		1.0
Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Debt securities	0.0		0.0
Significant unobservable inputs (Level 3) | Residential mortgage-backed securities
Assets
Debt securities	0.0		0.0
Significant unobservable inputs (Level 3) | Commercial mortgage-backed securities
Assets
Debt securities	0.0
Significant unobservable inputs (Level 3) | Bank debt
Assets
Debt securities			0.0
Significant unobservable inputs (Level 3) | Corporate debt securities
Assets
Debt securities	0.0		0.0
Significant unobservable inputs (Level 3) | U.S. government and government agency
Assets
Debt securities	0.0		0.0
Liabilities
Securities sold, not yet purchased			0.0
Significant unobservable inputs (Level 3) | Non-U.S. government and government agency
Assets
Debt securities	0.0
Significant unobservable inputs (Level 3) | U.S. states, municipalities and political subdivision
Assets
Debt securities	0.0
Significant unobservable inputs (Level 3) | Preferred stocks
Assets
Debt securities	2.8
Significant unobservable inputs (Level 3) | Fixed income mutual funds
Assets
Equity securities	0.0
Significant unobservable inputs (Level 3) | Common stocks
Assets
Equity securities	0.0
Valued at NAV
Assets
Investments in funds valued at NAV	$ 1,028.8		$ 1,055.6