Significant Transactions (Dispositions) (Details) - USD ($) $ / shares in Units, $ in Millions	2 Months Ended	3 Months Ended		12 Months Ended
	Jun. 30, 2016	Dec. 31, 2014	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jul. 24, 2015	Dec. 23, 2014
Significant Transactions
Gain (loss) on sale of discontinued operations, net of tax				$ 18.2	$ (1.6)	$ 46.6
Gain (Loss) on Disposition of Stock in Subsidiary				16.1	0.0	21.3
Premiums Written, Net				$ 1,172.6	$ 1,239.0	$ 1,102.2
Dividends declared and paid per White Mountains’s common share				$ 1.00	$ 1.00	$ 1.00
Total investments		$ 3,742.4		$ 4,280.7	$ 3,742.4
Symetra
Significant Transactions
Sale of Stock, Price Per Share				$ 32.00
Dividends declared and paid per White Mountains’s common share				$ 0.50
Sirius Group
Significant Transactions
Percentage of common shareholder's equity							127.30%
Additional cash that will be paid as part of the purchase price							$ 10.0
Total investments				$ 686.2
OneBeacon Runoff
Significant Transactions
Surplus notes								$ 101.0
Essentia | OneBeacon
Significant Transactions
Gain (Loss) on Disposition of Stock in Subsidiary			$ 23.0
Gain (Loss) on Disposition of Stock in Subsidiary, After Tax			$ 15.0
Gain (Loss) from Adjustments to Prior Period Extraordinary Items				3.7
Fair Value [Member] | Other long-term investments | OneBeacon Runoff
Significant Transactions
Surplus notes								$ 64.9
Discontinued Operations
Significant Transactions
Gain (loss) on sale of discontinued operations, net of tax						$ 46.6
Discontinued Operations | Sirius Group
Significant Transactions
Gain (loss) on sale of discontinued operations, net of tax						0.0
Total investments		3,142.7		2,972.6	3,142.7
Discontinued Operations | OneBeacon Runoff
Significant Transactions
Gain (loss) on sale of discontinued operations, net of tax				0.3	$ (18.8)	$ 46.6
Surplus Note | Discontinued Operations | OneBeacon Runoff
Significant Transactions
Gain (loss) on sale of discontinued operations, net of tax		$ 36.1
Crop
Significant Transactions
Proceeds related from termination of agreement				$ 3.0
Sale of Sirius | UNITED STATES
Significant Transactions
Industry Loss Warranty Limit on Contract	$ 45.0
Industry Loss Trigger on Industry Loss Warranty	15,000.0
Sale of Sirius | JAPAN
Significant Transactions
Industry Loss Warranty Limit on Contract	25.0
Industry Loss Trigger on Industry Loss Warranty	12,500.0
Combination One [Member] | Sale of Sirius | UNITED STATES
Significant Transactions
Industry Loss Warranty Limit on Contract	75.0
Industry Loss Trigger on Industry Loss Warranty	40,000.0
Combination Two [Member] | Sale of Sirius | UNITED STATES
Significant Transactions
Industry Loss Warranty Limit on Contract	22.5
Industry Loss Trigger on Industry Loss Warranty	$ 50,000.0