SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Total assets	$ 3,983.2	$ 3,362.6
Liabilities
Payable to subsidiary	6.6	4.0	$ 11.6
Total liabilities	838.5	644.4
White Mountains’s common shareholders’ equity	3,144.7	2,718.2
Non-controlling interests	(116.8)	(124.9)
Total liabilities and equity	3,983.2	3,362.6
Fair value investments	2,746.0	2,328.7	3,208.3
White Mountains Insurance Group Ltd.
Assets
Cash	0.7	0.7
Fixed maturity investments, at fair value	10.0	0.0
Common equity securities, at fair value	193.5	335.6
Short-term investments, at amortized cost	66.2	28.0
Other assets	1.9	1.8
Investments in consolidated subsidiaries	2,969.0	2,533.2
Total assets	3,241.3	2,899.3
Liabilities
Payable to subsidiary	(69.3)	15.8
Other liabilities	34.7	25.9
Total liabilities	(34.6)	41.7
White Mountains’s common shareholders’ equity	3,261.5	2,843.1
Non-controlling interests	14.4	14.5
Total liabilities and equity	3,241.3	2,899.3
Sirius Group
Assets
Cash	0.0	0.0	$ 0.0	$ 70.5
Sirius Group | Swedish Tax Authority | Sirius Group
Liabilities
Loss contingency accrual	$ 16.5	$ 17.3