Commitments And Contingencies (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	1 Months Ended			6 Months Ended						0 Months Ended			0 Months Ended	6 Months Ended
	Jun. 25, 2008	May 31, 2011	Oct. 31, 2010	Sep. 30, 2010	Jul. 31, 2011	Nov. 15, 2011	Jan. 31, 2011	Jul. 31, 2010	Dec. 31, 2009	Jul. 31, 2006	Oct. 18, 2011 Settlement Agreements [Member]	Jul. 31, 2011 Settlement Agreements [Member]	Jan. 31, 2011 Settlement Agreements [Member]	Jul. 20, 2011 First Condition [Member]	Jul. 31, 2011 First Condition [Member]	Jul. 31, 2011 Second Condition [Member]
Guarantee					$ 61.9
Forfeiture action against certain accounts										50.0
Aggregate amount to be paid under settlement agreement					165.0
Amount paid under settlement agreement		30.0				112.5		17.9	1.0
Total agreed payment in respect of anticipated opt out credit								21.5
Amount holdback in respect of anticipated opt out credit								3.6
Class action settlement fund payable in cash or stock					30.0
Aggregate value of shares delivered at any one time at minimum					27.5
Net cash proceeds from the sale of auction rate securities														50.0	50.0	100.0
Payment for settlement on net cash proceeds from the sale of auction rate securities											20.0				50.0	50.0
Cash received from sales and redemptions of auction rate securities					33.6		33.4
Restricted cash received from sales and redemptions of auction rate securities					33.6		33.4
Payment of restricted cash under settlement agreement		30.0
Amount deposited into the derivative settlement fund					60.0
Percentage of outstanding share capital					2.50%
Amount certain other defendants in the direct actions and the shareholder derivative actions have paid or agreed to pay					1.4
Legal fees and expenses of the plaintiffs			9.4
Insurance proceeds in connection with the settlements of the shareholder derivative actions and the consolidated shareholder class action				16.5
Litigation settlement												116.2	146.1
Monetary penalties under settlement agreement	0
Fine paid under non-prosecution agreement					1.2
Amount paid for disgorgement and pre-judgment interest					$ 1.6