Acquisition of Sirius Group - Narrative (Details)			10 Months Ended	12 Months Ended
	Feb. 26, 2021 USD ($) $ / shares shares	Aug. 06, 2020	Dec. 31, 2021 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Feb. 25, 2021 $ / shares shares
Business Acquisition [Line Items]
Common shares, par value (in dollars per share) | $ / shares			$ 0.10	$ 0.10
Dividends paid to preference shareholders				$ 12,200,000	$ 0	$ 0
Liability-classified capital instruments	$ 137,600,000		$ 87,800,000	87,800,000	$ 0
Number of consecutive trading days	30 days
VOBA carrying value			50,000,000	50,000,000
Amortization of VOBA			97,900,000
Liability-classified capital instruments
Business Acquisition [Line Items]
Gains (losses) from the change in fair value			49,800,000
Series A preference shares
Business Acquisition [Line Items]
Liability-classified capital instruments	$ 40,800,000		20,400,000	20,400,000
Series A preference shares | Liability-classified capital instruments
Business Acquisition [Line Items]
Gains (losses) from the change in fair value			20,400,000	20,400,000
Merger warrants
Business Acquisition [Line Items]
Liability-classified capital instruments	53,400,000		32,500,000	32,500,000
Merger warrants | Liability-classified capital instruments
Business Acquisition [Line Items]
Gains (losses) from the change in fair value			20,900,000	20,900,000
Private warrants
Business Acquisition [Line Items]
Liability-classified capital instruments	7,300,000		3,200,000	3,200,000
Private warrants | Liability-classified capital instruments
Business Acquisition [Line Items]
Gains (losses) from the change in fair value			4,100,000	4,100,000
Sirius Group Public Warrants
Business Acquisition [Line Items]
Liability-classified capital instruments	2,600,000		1,100,000	1,100,000
Sirius Group Public Warrants | Liability-classified capital instruments
Business Acquisition [Line Items]
Gains (losses) from the change in fair value			1,500,000	1,500,000
Upside Rights
Business Acquisition [Line Items]
Liability-classified capital instruments	6,500,000		0	0
Upside Rights | Liability-classified capital instruments
Business Acquisition [Line Items]
Gains (losses) from the change in fair value			6,500,000	6,500,000
CVRs
Business Acquisition [Line Items]
Liability-classified capital instruments	27,000,000.0		30,600,000	30,600,000
CVRs | Liability-classified capital instruments
Business Acquisition [Line Items]
Gains (losses) from the change in fair value			$ (3,600,000)	(3,600,000)
Affiliated entity
Business Acquisition [Line Items]
Consideration received	48,600,000
Maximum percentage of shares committed to purchase		9.50%
Sirius Group
Business Acquisition [Line Items]
Cash consideration	100,400,000
Loss and loss adjustment expense reserves, fair value adjustment	80,600,000
Loss and loss adjustment expense reserves, casualty liability adjustment	70,000,000
VOBA recognized as a result of the acquisition	147,900,000
Finite lived intangible assets recognized	130,000,000
Indefinite lived intangible assets recognized	$ 48,800,000
Sirius Group | Common shares
Business Acquisition [Line Items]
Shares issued (in shares) | shares	58,331,196
Value of shares issued	$ 595,600,000
Sirius Group | CVRs
Business Acquisition [Line Items]
Contingent cash payment per share contributor (in dollars per share) | $ / shares	$ 13.73
Settlement calculation, number of consecutive trading days	14 days
Value of shares issued	$ 27,000,000.0
Settlement calculation, multiplier	0.743
Sirius Group | Series A preference shares
Business Acquisition [Line Items]
Shares issued (in shares) | shares	11,720,987
Value of shares issued	$ 40,800,000
Par value (in dollars per share) | $ / shares	$ 0.10
Dividends paid to preference shareholders				$ 0
TPRE Net COVID Loss threshold	$ 51,100,000
Sirius Net COVID Loss threshold	150,000,000
TPRE Net COVID Loss	0
Sirius Net COVID Loss	0
Numerator for shares forfeiture calculation where the Sirius Net COVID Loss is greater than the TPRE Net COVID Loss, maximum amount	$ 100,000,000
Denominator for shares forfeiture calculation where the Sirius Net COVID Loss is greater than the TPRE Net COVID Loss, number of business days	30 days
Denominator for shares forfeiture calculation where the TPRE Net COVID Loss is greater than the Sirius Net COVID Loss, number of trading days	30 days
Convertible preferred stock, shares issued upon conversion (in shares) | shares	1
Sirius Group | Merger warrants
Business Acquisition [Line Items]
Value of shares issued	$ 53,400,000
Number of shares permitted to purchase per warrant (in shares) | shares	1
Warrant exercise price (in dollars per share) | $ / shares	$ 11.00
Warrants, term	5 years
Number of shares in which the warrants can be converted (in shares) | shares			21,009,324	21,009,324
Sirius Group | Private warrants
Business Acquisition [Line Items]
Value of shares issued	$ 7,300,000
Warrant exercise price (in dollars per share) | $ / shares	$ 13.00
Minimum equity and cash value per share (in dollars per share) | $ / shares	$ 13.73
Sirius Group | Upside Rights
Business Acquisition [Line Items]
Value of shares issued	$ 6,500,000
Target share price (in dollars per share) | $ / shares	$ 20.00
Numerator for settlement calculation, common shares value	$ 100,070,726
Denominator for settlement calculation, number of days	30 days
Sirius Group | Consideration Option One
Business Acquisition [Line Items]
Cash paid per acquiree share (in dollars per share) | $ / shares	$ 9.50
Sirius Group | Consideration Option Two
Business Acquisition [Line Items]
Common shares, par value (in dollars per share) | $ / shares	$ 0.10
Sirius Group | Consideration Option Two | Common shares
Business Acquisition [Line Items]
Entity shares issued per acquiree share (in shares) | shares	0.743
Sirius Group | Consideration Option Two | CVRs
Business Acquisition [Line Items]
Entity shares issued per acquiree share (in shares) | shares	1
Contingent cash payment per share contributor (in dollars per share) | $ / shares	$ 13.73
Contingent cash payment, threshold share price (in dollars per share) | $ / shares	$ 18.50
Settlement calculation, number of consecutive trading days	14 days
Shares issued (in shares) | shares	4,700,000
Sirius Group | Consideration Option Three
Business Acquisition [Line Items]
Cash paid per acquiree share (in dollars per share) | $ / shares	$ 0.905
Entity principal amount of shares issued per acquiree share	$ 0.905
Sirius Group | Consideration Option Three | Common shares
Business Acquisition [Line Items]
Entity shares issued per acquiree share (in shares) | shares	0.496
Sirius Group | Consideration Option Three | Series A preference shares
Business Acquisition [Line Items]
Entity shares issued per acquiree share (in shares) | shares	0.106
Preference shares, par value (in dollars per share) | $ / shares	$ 0.10
Sirius Group | Consideration Option Three | Merger warrants
Business Acquisition [Line Items]
Entity shares issued per acquiree share (in shares) | shares	0.190
Sirius Group
Business Acquisition [Line Items]
Common shares, par value (in dollars per share) | $ / shares							$ 0.01
Sirius Group | Sirius Group Public Warrants
Business Acquisition [Line Items]
Warrant exercise price (in dollars per share) | $ / shares	$ 18.89
Sirius Group | Private warrants
Business Acquisition [Line Items]
Number of shares in which the warrants can be converted (in shares) | shares							5,418,434