Selected Quarterly Financial Data (Unaudited) Selected Quarterly Financial Data Revised (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
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenues		$ 87.5	$ 83.5	$ 88.8	$ (5.5)	$ 56.8	$ 48.4	$ 58.0	$ 373.8	$ 157.7	$ 440.0
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 (expense) benefit	2.5	4.0	1.0	0.3	10.2	17.1	4.0	1.6	7.8	32.9	(12.7)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	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
Net income attributable to White Mountains’s common shareholders (3)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (38.9)	$ 90.8	$ 340.4	$ 9.5	$ 627.2	$ 401.8	$ 295.2
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
Earnings Per Share, Basic	6.00	130.81	2.97	6.36	(8.52)	18.65	66.80	1.70	146.06	80.11	50.20
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
Diluted earnings per share (in dollars) - discontinued operations:	$ 6.00	$ 130.81	$ 2.97	$ 6.36	$ (8.52)	$ 18.61	$ 66.64	$ 1.70	$ 146.06	$ 80.06	$ 50.20
Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenues			$ 85.4	$ 93.5	$ 0.3		$ 49.2	$ 58.8		$ 166.3	$ 435.3
Expenses			85.8	92.2	69.1		74.3	88.5		302.8	303.3
Pre-tax (loss) income from continuing operations			(0.4)	1.3	(68.8)		(25.1)	(29.7)		(136.5)	132.0
Income Tax (expense) benefit			1.0	0.3	10.2		4.0	1.6		32.9	(12.7)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest			0.6	1.6	(58.6)		(21.1)	(28.1)		(103.6)	119.3
Net (loss) income from discontinued operations, net of tax			2.8	31.3	8.0		383.6	47.4		108.3	116.9
Non-controlling interest in consolidated subsidiaries			12.1	1.3	17.3		(21.4)	(6.3)		(7.3)	18.1
Net income attributable to White Mountains’s common shareholders (3)			$ 15.5	$ 34.2	$ (33.3)		$ 341.1	$ 13.0		412.5	297.6
Continuing operations			$ 2.78	$ 0.65	$ (9.04)		$ (8.34)	$ (6.22)
Discontinued operations			0.61	6.86	1.75		75.27	8.55
Earnings Per Share, Basic			3.39	7.51	(7.29)		66.93	2.33
Continuing operations			2.78	0.65	(9.04)		(8.34)	(6.22)
Discontinued operations			0.61	6.86	1.75		75.11	8.55
Diluted earnings per share (in dollars) - discontinued operations:			$ 3.39	$ 7.51	$ (7.29)		$ 66.77	$ 2.33
Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenues			$ (1.9)	$ (4.7)	$ (5.8)		$ (0.8)	$ (0.8)		(8.6)	4.7
Expenses			(0.1)	0.2	(0.1)		(0.2)	2.8		2.2	7.5
Pre-tax (loss) income from continuing operations			(1.8)	(4.9)	(5.7)		(0.6)	(3.6)		(10.8)	(2.8)
Income Tax (expense) benefit				0.0	0.0		0.0	0.0		0.0	0.0
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest			(1.8)	(4.9)	(5.7)		(0.6)	(3.6)		(10.8)	(2.8)
Net (loss) income from discontinued operations, net of tax			0.0	0.0	0.0		0.0	0.0		0.0	0.0
Non-controlling interest in consolidated subsidiaries			(0.1)	(0.3)	0.1		(0.1)	0.1		0.1	0.4
Net income attributable to White Mountains’s common shareholders (3)			$ (1.9)	$ (5.2)	$ (5.6)		$ (0.7)	$ (3.5)		$ (10.7)	$ (2.4)
Continuing operations			$ (0.42)	$ (1.15)	$ (1.23)		$ (0.13)	$ (0.63)
Discontinued operations			0.00	0.00	0.00		0.00	0.00
Earnings Per Share, Basic			(0.42)	(1.15)	(1.23)		(0.13)	(0.63)
Continuing operations			(0.42)	(1.15)	(1.23)		(0.13)	(0.63)
Discontinued operations			0.00	0.00	0.00		0.00	0.00
Diluted earnings per share (in dollars) - discontinued operations:			$ (0.42)	$ (1.15)	$ (1.23)		$ (0.13)	$ (0.63)