Investment Securities (Cost and Amortized Cost Maturity Schedule) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Trading Securities by Maturity Cost or Amortized Cost
Cost or amortized cost	$ 4,664.8wtm_TradingSecuritiesAmortizedCost
Trading Securities by Maturity, Fair Value
Trading Securities	4,796.5us-gaap_TradingSecurities
Fixed Maturities and Convertible Fixed Maturities Excluding A B S M B S and Preferred Stock
Trading Securities by Maturity Cost or Amortized Cost
Due in one year or less	331.2
Due after one year through five years	1,968.3
Due after five years through ten years	434.4
Due after ten years	40.2
Trading Securities by Maturity, Fair Value
Due in one year or less	337.5
Due after one year through five years	2,032.3
Due after five years through ten years	453.9
Due after ten years	46.1
Mortgage-backed and asset-backed securities
Trading Securities by Maturity Cost or Amortized Cost
Trading Securities, Maturities without Single Maturity Date, Amortized Cost	1,811.1
Trading Securities by Maturity, Fair Value
Trading Securities, Maturities without Single Maturity Date, Carrying Value	1,840.9
Preferred stocks
Trading Securities by Maturity Cost or Amortized Cost
Trading Securities, Maturities without Single Maturity Date, Amortized Cost	79.6
Trading Securities by Maturity, Fair Value
Trading Securities, Maturities without Single Maturity Date, Carrying Value	$ 85.8

Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.4)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

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This item represents the cost of trading securities, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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This item represents securities, at cost, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments, as defined), and fair value hedge accounting adjustments, if any, which are expected to mature after five years and through ten years from the balance sheet date classified as trading.

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This item represents the carrying value of debt securities which are expected to mature after five years and through ten years from the balance sheet date classified as trading.

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This item represents securities, at cost, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments, as defined), and fair value hedge accounting adjustments, if any, which are expected to mature after one year and through five years from the balance sheet date classified as trading.

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This item represents the carrying value of securities which are expected to mature after one year and through five years from the balance sheet classified as trading.

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This item represents the carrying value of debt securities which are expected to mature after ten years from the balance sheet date classified as trading.

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This item represents securities, at cost, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments, as defined), and fair value hedge accounting adjustments, if any, which are expected to mature within one year of the balance sheet date classified as trading.

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This item represents the carrying value of securities which are expected to mature within one year of the balance sheet date classified as trading.

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This item represents the cost or amortized cost of securities, categorized as trading securities, which do not have a single maturity date and which the Company has decided to disclose separately, rather than allocating such cost over several maturity groupings.

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