Financial Statement Revisions - Statement of Operations and Comprehensive Income (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Revenues:
Advertising and commission revenues	$ 71.0		$ 33.8
Total revenues	42.1		88.8
Expenses:
Total expenses	108.1		92.4
Pre-tax income (loss) from continuing operations	(66.0)		(3.6)
Income tax benefit	(0.7)		0.3
Net income (loss) from continuing operations	(66.7)		(3.3)
Gain from sale of other discontinued operations, net of tax	0.1	$ 0.1	(1.0)
Net (loss) income from discontinued operations, net of tax	0.0		32.3
Net income	(66.6)		28.0
Net loss (income) attributable to non-controlling interests	18.6		1.0
Net income attributable to White Mountains’s common shareholders	(48.0)		29.0
Other comprehensive income, net of tax:
Other comprehensive income (loss), net of tax			0.2
Comprehensive income	(48.0)		29.2
Other comprehensive income attributable to non-controlling interests	0.0		0.0
Comprehensive income attributable to White Mountains’s common shareholders	(48.0)		$ 29.2
Continuing operations (in usd per share)			$ 6.34
As previously reported
Revenues:
Total revenues			$ 93.5
Expenses:
Total expenses			92.2
Pre-tax income (loss) from continuing operations			1.3
Income tax benefit			0.3
Net income (loss) from continuing operations			1.6
Gain from sale of other discontinued operations, net of tax			(1.0)
Net (loss) income from discontinued operations, net of tax			32.3
Net income			32.9
Net loss (income) attributable to non-controlling interests			1.3
Net income attributable to White Mountains’s common shareholders			34.2
Other comprehensive income, net of tax:
Other comprehensive income (loss), net of tax			0.9
Comprehensive income			35.1
Other comprehensive income attributable to non-controlling interests			0.0
Comprehensive income attributable to White Mountains’s common shareholders			$ 35.1
Continuing operations (in usd per share)			$ 7.49
Adjustments
Revenues:
Total revenues			$ (4.7)
Expenses:
Total expenses			0.2
Pre-tax income (loss) from continuing operations			(4.9)
Income tax benefit			0.0
Net income (loss) from continuing operations			(4.9)
Gain from sale of other discontinued operations, net of tax			0.0
Net (loss) income from discontinued operations, net of tax			0.0
Net income			(4.9)
Net loss (income) attributable to non-controlling interests			(0.3)
Net income attributable to White Mountains’s common shareholders			(5.2)	$ (5.2)
Other comprehensive income, net of tax:
Other comprehensive income (loss), net of tax			(0.7)
Comprehensive income			(5.9)
Other comprehensive income attributable to non-controlling interests			0.0
Comprehensive income attributable to White Mountains’s common shareholders			$ (5.9)
Continuing operations (in usd per share)			$ (1.15)
HG Global-BAM
Revenues:
Total revenues	(1.0)		$ 6.3
Expenses:
Total expenses	13.3		11.9
HG Global-BAM | Adjustments
Revenues:
Total revenues			0.0
Expenses:
Total expenses			0.0
MediaAlpha
Revenues:
Advertising and commission revenues	70.1		32.5
Total revenues	71.7		32.5
Expenses:
Total expenses	71.9		33.5
MediaAlpha | As previously reported
Revenues:
Total revenues			32.5
Expenses:
Total expenses			33.5
MediaAlpha | Adjustments
Revenues:
Total revenues			0.0
Expenses:
Total expenses			0.0
Other Entity
Revenues:
Advertising and commission revenues	0.9		1.3
Other revenues			48.7
Total revenues	(28.6)		50.0
Expenses:
Total expenses	22.9		47.0
Other Entity | As previously reported
Revenues:
Advertising and commission revenues			6.0
Expenses:
Total expenses			46.8
Other Entity | Adjustments
Revenues:
Advertising and commission revenues			(4.7)
Other revenues			0.0
Expenses:
Total expenses			0.2
HG Global-BAM
Revenues:
Advertising and commission revenues	0.0		0.0
Expenses:
Pre-tax income (loss) from continuing operations	$ (14.3)		(5.6)
HG Global-BAM | As previously reported
Revenues:
Total revenues			6.3
Expenses:
Total expenses			$ 11.9