Financial Statement Revisions - Statement of Operations and Comprehensive Income (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues:
Earned insurance premiums	$ 2.4	$ 3.4		$ 7.6	$ 10.2
Net investment income	12.2	9.6		39.7	18.2
Net realized and unrealized investment gains	32.5	10.9		102.5	27.2
Advertising and commission revenues	38.8	28.2		103.9	89.6
Other revenue	1.6	4.7		6.1	18.0
Total revenues	87.5	56.8		259.8	163.2
Expenses:
Loss and loss adjustment expenses	0.0	2.2		1.1	6.8
Insurance acquisition expenses	0.9	1.3		3.1	4.4
Other underwriting expenses	0.1	0.2		0.3	0.4
Cost of sales	33.1	24.0		88.7	76.9
General and administrative expenses	44.1	42.4		162.2	144.9
Interest expense	0.9	0.5		1.8	2.6
Total expenses	79.1	70.6		257.2	236.0
Pre-tax income (loss) from continuing operations	8.4	(13.8)		2.6	(72.8)
Income tax benefit	4.0	17.1		5.3	22.7
Net income (loss) from continuing operations	12.4	3.3		7.9	(50.1)
Gain from sale of other discontinued operations, net of tax	554.3	47.9		552.7	414.5
Net (loss) income from discontinued operations, net of tax	(15.2)	36.5		20.5	100.9
Net income	551.5	87.7		581.1	465.3
Net loss (income) attributable to non-controlling interests	10.6	3.1		23.6	(24.6)
Net income attributable to White Mountains’s common shareholders	562.1	90.8		604.7	440.7
Comprehensive income, net of tax:
Comprehensive income	565.1	90.7		608.1	586.1
Other comprehensive income attributable to non-controlling interests	0.0	0.1		(0.1)	0.1
Comprehensive income attributable to White Mountains’s common shareholders	$ 565.1	$ 90.8		$ 608.0	$ 586.2
Continuing operations (in usd per share)	$ 5.36	$ 1.31		$ 7.03	$ (14.47)
As previously reported
Revenues:
Earned insurance premiums		$ 3.4	$ 5.2		$ 10.2	$ 13.4	$ 12.0	$ 7.9
Net investment income		9.6	27.5		18.2	32.1	10.9	12.2
Net realized and unrealized investment gains		10.9	70.0		27.2	(27.4)	260.5	38.1
Advertising and commission revenues		29.4	71.7		92.4	126.9	110.1	65.7
Other revenue		4.7	4.5		18.0	21.3	41.8	20.5
Total revenues		58.0	178.9		166.0	166.3	435.3	144.4
Expenses:
Loss and loss adjustment expenses		2.2	1.1		6.8	8.0	8.2	8.9
Insurance acquisition expenses		1.3	2.2		4.4	5.6	6.3	2.9
Other underwriting expenses		0.2	0.2		0.4	0.5	0.4	0.4
Cost of sales		24.0	55.6		76.9	102.0	93.6	57.8
General and administrative expenses		42.7	117.9		142.6	183.7	193.2	144.9
Interest expense		0.5	0.9		2.6	3.0	1.6	1.2
Total expenses		70.9	177.9		233.7	302.8	303.3	216.1
Pre-tax income (loss) from continuing operations		(12.9)	1.0		(67.7)	(136.5)	132.0	(71.7)
Income tax benefit		17.1	1.3		22.7	32.9	(12.7)	2.5
Net income (loss) from continuing operations		4.2	2.3		(45.0)	(103.6)	119.3	(69.2)
Gain from sale of other discontinued operations, net of tax		47.9	(1.6)		414.5	415.1	17.9	17.2
Net (loss) income from discontinued operations, net of tax		36.5	35.7		100.9	108.3	117.2	296.4
Income (loss) before equity in earnings of unconsolidated affiliates						419.8	254.4	244.4
Equity in earnings of unconsolidated affiliates						0.0	25.1	45.6
Net income		88.6	36.4		470.4	419.8	279.5	290.0
Net loss (income) attributable to non-controlling interests		3.1	13.4		(24.6)	(7.3)	18.1	22.2
Net income attributable to White Mountains’s common shareholders		91.7	49.8		445.8	412.5	297.6	312.2
Comprehensive income, net of tax:
Other comprehensive income		0.2	1.7		145.6	145.6	(100.4)	(104.9)
Comprehensive income		91.9	51.5		591.4	558.1	197.2	207.3
Other comprehensive income attributable to non-controlling interests		0.1	(0.1)		0.1	(0.3)	0.0	3.3
Comprehensive income attributable to White Mountains’s common shareholders		$ 92.0	$ 51.4		$ 591.5	$ 557.8	$ 197.2	$ 210.6
Continuing operations (in usd per share)		$ 1.50	$ 0		$ (13.49)	$ 0	$ 0	$ 0
Adjustments
Revenues:
Earned insurance premiums		$ 0.0	$ 0.0		$ 0.0	$ 0.0	$ 0.0	$ 0.0
Net investment income		0.0	0.0		0.0	0.0	0.0	0.0
Net realized and unrealized investment gains		0.0	0.0		0.0	0.0	0.0	0.0
Advertising and commission revenues		(1.2)	(6.6)		(2.8)	(8.6)	(2.7)	(0.2)
Other revenue		0.0	0.0		0.0	0.0	7.4	(6.9)
Total revenues		(1.2)	(6.6)		(2.8)	(8.6)	4.7	(7.1)
Expenses:
Loss and loss adjustment expenses		0.0	0.0		0.0	0.0	0.0	0.0
Insurance acquisition expenses		0.0	0.0		0.0	0.0	0.0	0.0
Other underwriting expenses		0.0	0.0		0.0	0.0	0.0	0.0
Cost of sales		0.0	0.0		0.0	0.0	0.0	0.0
General and administrative expenses		(0.3)	0.1		2.3	2.2	7.5	0.0
Interest expense		0.0	0.0		0.0	0.0	0.0	0.0
Total expenses		(0.3)	0.1		2.3	2.2	7.5	0.0
Pre-tax income (loss) from continuing operations		(0.9)	(6.7)		(5.1)	(10.8)	(2.8)	(7.1)
Income tax benefit		0.0	0.0		0.0	0.0	0.0	0.0
Net income (loss) from continuing operations		(0.9)	(6.7)		(5.1)	(10.8)	(2.8)	(7.1)
Gain from sale of other discontinued operations, net of tax		0.0	0.0		0.0	0.0	0.0	0.0
Net (loss) income from discontinued operations, net of tax		0.0	0.0		0.0	0.0	0.0	0.0
Income (loss) before equity in earnings of unconsolidated affiliates						(10.8)	(2.8)	(7.1)
Equity in earnings of unconsolidated affiliates							0.0	0.0
Net income		(0.9)	(6.7)		(5.1)	(10.8)	(2.8)	(7.1)
Net loss (income) attributable to non-controlling interests		0.0	(0.4)		0.0	0.1	0.4	0.1
Net income attributable to White Mountains’s common shareholders		(0.9)	(7.1)		(5.1)	(10.7)	(2.4)	(7.0)
Comprehensive income, net of tax:
Other comprehensive income		(0.2)	(1.3)		(0.2)	0.0	(7.4)	6.9
Comprehensive income		(1.1)	(8.4)		(5.3)	(10.7)	(9.8)	(0.1)
Other comprehensive income attributable to non-controlling interests		0.0	0.0		0.0	0.0	0.0	0.0
Comprehensive income attributable to White Mountains’s common shareholders		$ (1.1)	$ (8.4)		$ (5.3)	$ (10.7)	$ (9.8)	$ (0.1)
Continuing operations (in usd per share)		$ (0.18)	$ 0		$ (1)	$ 0	$ 0	$ 0
As revised
Revenues:
Earned insurance premiums		$ 3.4	$ 5.2		$ 10.2	$ 13.4	$ 12.0	$ 7.9
Net investment income		9.6	27.5		18.2	32.1	10.9	12.2
Net realized and unrealized investment gains		10.9	70.0		27.2	(27.4)	260.5	38.1
Advertising and commission revenues		28.2	65.1		89.6	118.3	107.4	65.5
Other revenue		4.7	4.5		18.0	21.3	49.2	13.6
Total revenues		56.8	172.3		163.2	157.7	440.0	137.3
Expenses:
Loss and loss adjustment expenses		2.2	1.1		6.8	8.0	8.2	8.9
Insurance acquisition expenses		1.3	2.2		4.4	5.6	6.3	2.9
Other underwriting expenses		0.2	0.2		0.4	0.5	0.4	0.4
Cost of sales		24.0	55.6		76.9	102.0	93.6	57.8
General and administrative expenses		42.4	118.0		144.9	185.9	200.7	144.9
Interest expense		0.5	0.9		2.6	3.0	1.6	1.2
Total expenses		70.6	178.0		236.0	305.0	310.8	216.1
Pre-tax income (loss) from continuing operations		(13.8)	(5.7)		(72.8)	(147.3)	129.2	(78.8)
Income tax benefit		17.1	1.3		22.7	32.9	(12.7)	2.5
Net income (loss) from continuing operations		3.3	(4.4)		(50.1)	(114.4)	116.5	(76.3)
Gain from sale of other discontinued operations, net of tax		47.9	(1.6)		414.5	415.1	17.9	17.2
Net (loss) income from discontinued operations, net of tax		36.5	35.7		100.9	108.3	117.2	296.4
Income (loss) before equity in earnings of unconsolidated affiliates						409.0	251.6	237.3
Equity in earnings of unconsolidated affiliates						0.0	25.1	45.6
Net income		87.7	29.7		465.3	409.0	276.7	282.9
Net loss (income) attributable to non-controlling interests		3.1	13.0		(24.6)	(7.2)	18.5	22.3
Net income attributable to White Mountains’s common shareholders		90.8	42.7		440.7	401.8	295.2	305.2
Comprehensive income, net of tax:
Other comprehensive income		0.0	0.4		145.4	145.6	(107.8)	(98.0)
Comprehensive income		90.8	43.1		586.1	547.4	187.4	207.2
Other comprehensive income attributable to non-controlling interests		0.1	(0.1)		0.1	(0.3)	0.0	3.3
Comprehensive income attributable to White Mountains’s common shareholders		$ 90.9	$ 43.0		$ 586.2	$ 547.1	$ 187.4	$ 210.5
Continuing operations (in usd per share)		$ 1.32	$ 0		$ (14.49)	$ 0	$ 0	$ 0