Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
(Loss) revenues (including realized gains and losses)	$ 268.2	$ 365.0	$ 358.2	$ 369.3	$ 602.6	$ 296.1	$ 368.7	$ 354.3	$ 1,360.7	$ 1,621.7	$ 1,411.1
Total expenses	335.0	345.0	360.3	360.8	356.3	383.0	373.8	351.3	1,401.1	1,464.4	1,440.5
Pre-tax (loss) income from continuing operations	(66.8)	20.0	(2.1)	8.5	246.3	(86.9)	(5.1)	3.0	(40.4)	157.3	(29.4)
Income Tax Expense (Benefit)	23.0	6.7	6.0	9.7	1.0	1.6	2.2	(4.6)	45.4	0.2	14.8
Net income (loss) from continuing operations	(43.8)	26.7	3.9	18.2	247.3	(85.3)	(2.9)	(1.6)	5.0	157.5	(14.6)
Net (loss) income from discontinued operations, net of tax	(6.8)	61.9	358.6	1.1	19.6	6.5	(0.5)	71.3	(0.3)	78.7	260.6
Non-controlling interest in consolidated subsidiaries	17.3	3.1	(21.4)	(6.3)	(6.1)	16.0	0.9	7.3	(7.3)	18.1	22.2
Equity in earnings of unconsolidated affiliates, net of tax	0.0	0.0	0.0	0.0	7.1	3.9	6.8	7.3	0.0	25.1	45.6
Net income attributable to White Mountains’s common shareholders	$ (33.3)	$ 91.7	$ 341.1	$ 13.0	$ 267.9	$ (58.9)	$ 4.3	$ 84.3	$ 412.5	$ 297.6	$ 312.2
Basic earnings per share
Continuing operations	$ (5.81)	$ 6.12	$ (3.44)	$ 2.51	$ 43.82	$ (11.10)	$ 0.80	$ 1.76	$ (0.47)	$ 34.12	$ 8.70
Discontinued operations	(1.49)	12.72	70.37	(0.17)	3.46	1.09	(0.08)	12.33	82.71	16.48	42.43
Total consolidated operations	(7.30)	18.84	66.93	2.34	47.28	(10.01)	0.72	14.09	82.24	50.60	51.13
Diluted earnings per share
Continuing operations	(5.81)	6.11	(3.43)	2.51	43.79	(11.10)	0.80	1.76	(0.47)	34.12	8.70
Discontinued operations	(1.49)	12.69	70.22	(0.17)	3.46	1.09	(0.08)	12.33	82.66	16.48	42.43
Total consolidated operations	$ (7.30)	$ 18.80	$ 66.79	$ 2.34	$ 47.25	$ (10.01)	$ 0.72	$ 14.09	$ 82.19	$ 50.60	$ 51.13

The total amount of expense recognized during the period for future policy benefits, claims and claims adjustment costs, and for selling, general and administrative costs.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 55
-Paragraph 52
-URI http://asc.fasb.org/extlink&oid=65016374&loc=d3e4984-109258

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(19))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64863997&loc=d3e1252-109256

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(21))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(23))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882