Aggregate dividends declared during the period for each share of common stock outstanding.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’ common shareholders									$ 316.1	$ 317.3	$ 322.4
Basic and diluted earnings per share numerators (in millions):
Allocation of income for unvested restricted common shares(1)									(4.1)	(4.7)	(4.3)
Dividends declared on participating restricted common shares(1)									(0.1)us-gaap_DividendsShareBasedCompensation	(0.1)us-gaap_DividendsShareBasedCompensation	(0.1)us-gaap_DividendsShareBasedCompensation
Total allocation to restricted common shares									(4.2)	(4.8)	(4.4)
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts									311.9	312.5	318.0
Undistributed net earnings (in millions):
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts									311.9	312.5	318.0
Dividends declared net of restricted common share amounts(1)									(6.1)	(6.1)	(6.5)
Total undistributed net earnings, net of restricted common share amounts									$ 305.8us-gaap_UndistributedEarnings	$ 306.4us-gaap_UndistributedEarnings	$ 311.5us-gaap_UndistributedEarnings
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									6,104,900	6,200,400	6,799,800
Average unvested restricted common shares									(78,900)	(91,400)	(91,100)
Basic earnings per share denominator									6,026,000	6,109,000	6,708,700
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period									6,104,900	6,200,400	6,799,800
Average unvested restricted common shares									(78,900)	(91,400)	(91,100)
Average outstanding dilutive options to acquire common shares									0	0	0
Diluted earnings (loss) per share denominator									6,026,000	6,109,000	6,708,700
Basic earnings per share (in dollars):
Net income attributable to White Mountains’s common shareholders	$ 13.71	$ 7.35	$ 15.10	$ 15.56	$ 19.14	$ 9.20	$ 3.62	$ 19.10	$ 51.77	$ 51.15	$ 47.41
Dividends declared and paid									$ (1.00)	$ (1.00)	$ (1.00)
Undistributed earnings									$ 50.77	$ 50.15	$ 46.41
Diluted earnings per share (in dollars)
Net income attributable to White Mountains’s common shareholders	$ 13.71	$ 7.35	$ 15.10	$ 15.56	$ 19.14	$ 9.20	$ 3.62	$ 19.10	$ 51.77	$ 51.15	$ 47.41
Dividends declared and paid									$ (1.00)	$ (1.00)	$ (1.00)
Undistributed earnings									$ 50.77	$ 50.15	$ 46.41
Incremental common shares attributable to share-based payment arrangements									125,000