Financial Statement Revisions - Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Assets
Investments	$ 3,280.5		$ 2,714.4
Cash	48.6		80.2
Insurance premiums receivable	4.5		1.6
Deferred acquisition costs	14.1		10.6
Accrued investment income	15.4		14.8
Accounts receivable on unsettled investment sales	190.1		4.8
Goodwill	25.9	$ 25.9	25.9	$ 25.9	$ 18.3	$ 18.6
Assets held for sale	0.0		3,599.5
Other assets	46.1		49.1
Total assets	3,637.1		6,520.2
Liabilities
Unearned insurance premiums	118.5		82.9
Debt	9.4		12.7
Accrued incentive compensation	76.9		95.7
Accounts payable on unsettled investment purchases	49.7		0.0
Liabilities held for sale	0.0		2,569.3
Other liabilities	45.4		43.6
Liabilities	299.9		2,804.2
White Mountains’s common shareholders’ equity
White Mountains’ common shares	3.7		4.6
Paid-in surplus	665.6		806.1
Retained earnings	2,800.8		2,776.6
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations	0.0		(3.2)
Total White Mountains’s common shareholders’ equity	3,468.8		3,582.7
Non-controlling interests	(131.6)		133.3
Total equity	3,337.2		3,716.0
Total liabilities and equity	$ 3,637.1		6,520.2
As previously reported
Assets
Investments		2,692.9	2,714.4			1,679.7
Cash		53.3	80.2			77.8
Insurance premiums receivable		2.8	1.6			1.3
Deferred acquisition costs		13.0	10.6			6.9
Deferred tax asset			0.0			0.0
Accrued investment income		16.0	14.8			3.8
Accounts receivable on unsettled investment sales		199.5	4.8			11.4
Goodwill		31.7	31.7			24.1
Intangible assets		17.8	23.0			28.9
Assets held for sale		3,696.4	3,599.5			8,365.6
Other assets		62.8	64.1			83.1
Total assets		6,786.2	6,544.7			10,282.6
Liabilities
Unearned insurance premiums		109.9	82.9			50.2
Debt		10.6	12.7			64.7
Deferred tax liability			0.0			27.4
Accrued incentive compensation		63.3	95.7			96.2
Accounts payable on unsettled investment purchases		114.6
Liabilities held for sale		2,678.8	2,569.3			5,618.1
Other liabilities		44.9	46.9			58.0
Liabilities		3,022.1	2,807.5			5,914.6
White Mountains’s common shareholders’ equity
White Mountains’ common shares		4.6	4.6			5.6
Paid-in surplus		810.5	806.1			972.6
Retained earnings		2,835.2	2,797.2			3,084.9
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations		(3.0)	(4.6)			(149.9)
Total White Mountains’s common shareholders’ equity		3,647.3	3,603.3			3,913.2
Non-controlling interests		116.8	133.9			454.8
Total equity		3,764.1	3,737.2			4,368.0
Total liabilities and equity		6,786.2	6,544.7			10,282.6
Adjustments
Assets
Investments		0.0	0.0			0.0
Cash		0.0	0.0			0.0
Insurance premiums receivable		0.0	0.0			0.0
Deferred acquisition costs		0.0	0.0			0.0
Deferred tax asset			0.0			0.0
Accrued investment income		0.0	0.0			0.0
Accounts receivable on unsettled investment sales		0.0	0.0			0.0
Goodwill		(5.8)	(5.8)			(5.5)
Intangible assets		(3.4)	(3.7)			(2.0)
Assets held for sale		0.0	0.0			0.0
Other assets		(19.7)	(15.0)			(3.9)
Total assets		(28.9)	(24.5)			(11.4)
Liabilities
Unearned insurance premiums		0.0	0.0			0.0
Debt		0.0	0.0			0.0
Deferred tax liability			0.0			0.0
Accrued incentive compensation		0.0	0.0			0.0
Accounts payable on unsettled investment purchases		0.0
Liabilities held for sale		0.0	0.0			0.0
Other liabilities		0.3	(3.3)			(1.0)
Liabilities		0.3	(3.3)			(1.0)
White Mountains’s common shareholders’ equity
White Mountains’ common shares		0.0	0.0			0.0
Paid-in surplus		0.7	0.0			0.0
Retained earnings		(27.7)	(20.6)			(9.9)
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations		(1.3)	0.0			0.0
Total White Mountains’s common shareholders’ equity		(28.3)	(20.6)			(9.9)
Non-controlling interests		(0.9)	(0.6)			(0.5)
Total equity		(29.2)	(21.2)			(10.4)
Total liabilities and equity		(28.9)	(24.5)			(11.4)
As revised
Assets
Investments		2,692.9	2,714.4			1,679.7
Cash		53.3	80.2			77.8
Insurance premiums receivable		2.8	1.6			1.3
Deferred acquisition costs		13.0	10.6			6.9
Deferred tax asset			0.0			0.0
Accrued investment income		16.0	14.8			3.8
Accounts receivable on unsettled investment sales		199.5	4.8			11.4
Goodwill		25.9	25.9			18.6
Intangible assets		14.4	19.3			26.9
Assets held for sale		3,696.4	3,599.5			8,365.6
Other assets		43.1	49.1			79.2
Total assets		6,757.3	6,520.2			10,271.2
Liabilities
Unearned insurance premiums		109.9	82.9			50.2
Debt		10.6	12.7			64.7
Deferred tax liability			0.0			27.4
Accrued incentive compensation		63.3	95.7			96.2
Accounts payable on unsettled investment purchases		114.6
Liabilities held for sale		2,678.8	2,569.3			5,618.1
Other liabilities		45.2	43.6			57.0
Liabilities		3,022.4	2,804.2			5,913.6
White Mountains’s common shareholders’ equity
White Mountains’ common shares		4.6	4.6			5.6
Paid-in surplus		811.2	806.1			972.6
Retained earnings		2,807.5	2,776.6			3,075.0
Accumulated other comprehensive loss, after tax:
Accumulated other comprehensive loss from net change in benefit plan assets and obligations		(4.3)	(4.6)			(149.9)
Total White Mountains’s common shareholders’ equity		3,619.0	3,582.7			3,903.3
Non-controlling interests		115.9	133.3			454.3
Total equity		3,734.9	3,716.0			4,357.6
Total liabilities and equity		$ 6,757.3	$ 6,520.2			$ 10,271.2