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Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended |  | 3 Months Ended | 9 Months Ended | 3 Months Ended
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Mar. 31, 2012 Restricted Share Cliff Vesting in January 2015 [Member] | Sep. 30, 2012 Restricted Share Cliff Vesting in January 2015 [Member] | Sep. 30, 2012 Restricted Share Cliff Vesting in July 2015 [Member] | Sep. 30, 2012 Restricted Stock [Member] | Sep. 30, 2011 Restricted Stock [Member] | Sep. 30, 2012 Restricted Stock [Member] | Sep. 30, 2011 Restricted Stock [Member] | Sep. 30, 2012 Restricted Share Vesting Beginning in February 2014 [Member] | Mar. 31, 2012 Restricted Share Vesting Beginning in February 2014 [Member] Installment | Mar. 31, 2011 Restricted Share Cliff Vesting in January 2014 [Member] | Sep. 30, 2012 Restricted Share Cliff Vesting in January 2014 [Member] | Mar. 31, 2010 Restricted Share Vested in December 2012 [Member]
Target Performance Share activity |  |  |  |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  |  |  | 93,460 | 73,500 | 72,000 | 46,250 |  |  |  |  |
Issued (in shares) | 25,460 | 2,500 | 1,200 | 3,700 | 0 | 32,160 | 27,250 | 3,000 |  | 27,000 | 250 | 19,750
Vested (in shares) |  |  |  | 0 | 0 | (7,000) | 0 |  |  |  |  |
Assumed forfeitures and cancellations |  |  |  | (1,065) | 0 | (1,065) | 0 |  |  |  |  |
Ending of period (in shares) |  |  |  | 96,095 | 73,500 | 96,095 | 73,500 |  |  |  |  |
Unamortized grant date fair value |  |  |  |  |  |  |  |  |  |  |  |
Unamortized grant date fair value, beginning balance |  |  |  | $ 21.5 | $ 18.9 | $ 13.3 | $ 14.1 |  |  |  |  |
Issued |  |  |  | 2.0 | 0 | 15.7 | 9.9 |  |  |  |  |
Assumed forfeitures and cancelations |  |  |  | (0.2) | 0 | (0.2) | 0 |  |  |  |  |
Expense recognized |  |  |  | (3.2) | (2.8) | (8.7) | (7.9) |  |  |  |  |
Unamortized grant date fair value, ending balance |  |  |  | $ 20.1 | $ 16.1 | $ 20.1 | $ 16.1 |  |  |  |  |
Number of restricted shares installments |  |  |  |  |  |  |  |  | 2 |  |  |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph g(1) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 64 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=6793087&loc=d3e301413-122809 + Details
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- Definition As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph h -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(e) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(d) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Number of share instruments newly issued under a share-based compensation plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 65 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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