Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effect of Fourth Quarter Events [Line Items]
Revenues (loss) (including realized gains and losses)		$ 87.5	$ 83.5	$ 88.8	$ (5.5)	$ 56.8	$ 48.4	$ 58.0	$ 373.8	$ 157.7	$ 440.0
Total expenses		79.1	85.7	92.4	69.0	70.6	74.1	91.3	366.0	305.0	310.8
Pre-tax (loss) income from continuing operations	$ 5.2	8.4	(2.2)	(3.6)	(74.5)	(13.8)	(25.7)	(33.3)	7.8	(147.3)	129.2
Income tax benefit (expense)	2.5	4.0	1.0	0.3	10.2	17.1	4.0	1.6	7.8	32.9	(12.7)
Net income (loss) from continuing operations	7.7	12.4	(1.2)	(3.3)	(64.3)	3.3	(21.7)	(31.7)	15.6	(114.4)	116.5
Net (loss) income from discontinued operations, net of tax	4.3	539.1	2.8	31.3	8.0	84.4	383.6	47.4	20.5	108.3	116.9
Non-controlling interest in consolidated subsidiaries	10.5	10.6	12.0	1.0	17.4	3.1	(21.5)	(6.2)	34.1	(7.2)	18.5
Equity in earnings of unconsolidated affiliates									0.0	0.0	25.1
Net income attributable to White Mountains’s common shareholders	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (38.9)	$ 90.8	$ 340.4	$ 9.5	$ 627.2	$ 401.8	$ 295.2
Basic earnings per share
Continuing operations	$ 4.85	$ 5.36	$ 2.36	$ (0.50)	$ (10.27)	$ 1.31	$ (8.47)	$ (6.85)	$ 11.56	$ (24.26)	$ 27.22
Discontinued operations	1.15	125.45	0.61	6.86	1.75	17.34	75.27	8.55	134.50	104.37	22.98
Total consolidated operations	6.00	130.81	2.97	6.36	(8.52)	18.65	66.80	1.70	146.06	80.11	50.20
Diluted earnings per share
Continuing operations	4.85	5.36	2.36	(0.50)	(10.27)	1.31	(8.47)	(6.85)	11.56	(24.26)	27.22
Discontinued operations	1.15	125.45	0.61	6.86	1.75	17.30	75.11	8.55	134.50	104.32	22.98
Total consolidated operations	$ 6.00	$ 130.81	$ 2.97	$ 6.36	$ (8.52)	$ 18.61	$ 66.64	$ 1.70	$ 146.06	$ 80.06	$ 50.20