Share Capital (Details) - Subsequent share repurchases (USD $) In Millions, unless otherwise specified	12 Months Ended	2 Months Ended	0 Months Ended
Dec. 31, 2012 February 2012 share repurchase plan	Feb. 25, 2013 Subsequent Event [Member] Repurchase of Equity [Member] February 2012 share repurchase plan	Feb. 22, 2013 Subsequent Event [Member] Repurchase of Equity [Member] February 2013 share repurchase plan
Subsequent Event [Line Items]
Share repurchases subsequent to year end		$ 98.4
Number of shares repurchased subsequent to year end		3.8
Amount remaining under share repurchase plan	348.4	250.0
Share repurchase authorization	$ 750.0		$ 850.0

Reflects the known or estimated financial effect of the event, or transaction that occurred between the balance sheet date and the date the financial statements are issued or available to be issued.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 855

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6842918&loc=SL6314017-165662

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 165

-Paragraph 13

-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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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