Debt (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Commercial Paper and Borrowings Under Bank Credit Facilities
Balance	$ 28.6	$ 115.6
Weighted average interest rate	1.26%	0.64%
Outstanding balances of debt obligations and the applicable interest rates
Carrying Value of debt	3,498.2	3,015.6
Debt discount	(9.3)	(14.2)
Debt adjustment for fair value hedges	15.3	17.6
Total Debt	3,504.2	3,019.0
Recourse Fixed Rate Debt Unsecured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	4/30/2009
Final Maturity	May 15, 2014
Fixed Interest Rate	8.75%
Carrying Value of debt	300.0	300.0
Recourse Fixed Rate Debt Unsecured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	9/24/2009
Final Maturity	Oct. 01, 2012
Fixed Interest Rate	4.75%
Carrying Value of debt	300.0	300.0
Recourse Fixed Rate Debt Unsecured Three [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	02/5/2010
Final Maturity	May 15, 2015
Fixed Interest Rate	4.75%
Carrying Value of debt	250.0	250.0
Recourse Fixed Rate Debt Unsecured Four [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	11/19/2010
Final Maturity	Jul. 15, 2016
Fixed Interest Rate	3.50%
Carrying Value of debt	250.0	250.0
Recourse Fixed Rate Debt Unsecured Five [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	5/27/2011
Final Maturity	Jun. 01, 2021
Fixed Interest Rate	4.85%
Carrying Value of debt	250.0	0
Recourse Fixed Rate Debt Unsecured Six [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	2/6/2008
Final Maturity	Feb. 15, 2018
Fixed Interest Rate	6.00%
Carrying Value of debt	200.0	200.0
Recourse Fixed Rate Debt Unsecured Seven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	03/03/2006
Final Maturity	Mar. 01, 2016
Fixed Interest Rate	5.80%
Carrying Value of debt	200.0	200.0
Recourse Fixed Rate Debt Unsecured Eight [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	10/11/2006
Final Maturity	Feb. 15, 2012
Fixed Interest Rate	5.50%
Carrying Value of debt	200.0	200.0
Recourse Fixed Rate Debt Unsecured Nine [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/22/2005
Final Maturity	Dec. 22, 2015
Fixed Interest Rate	5.75%
Carrying Value of debt	115.0	130.0
Recourse Fixed Rate Debt Unsecured Ten [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	9/20/2011
Final Maturity	Jul. 15, 2016
Fixed Interest Rate	3.50%
Carrying Value of debt	100.0	0
Recourse Fixed Rate Debt Unsecured Eleven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	4/14/2005
Final Maturity	Apr. 15, 2015
Fixed Interest Rate	5.70%
Carrying Value of debt	100.0	100.0
Recourse Fixed Rate Debt Unsecured Twelve [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	9/20/2011
Final Maturity	Jun. 01, 2021
Fixed Interest Rate	4.85%
Carrying Value of debt	50.0	0
Recourse Fixed Rate Debt Unsecured Thirteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	3/29/2006
Final Maturity	Dec. 11, 2012
Fixed Interest Rate	3.49%
Carrying Value of debt	40.5	41.8
Recourse Fixed Rate Debt Unsecured Fourteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/18/2007
Final Maturity	Nov. 30, 2012
Fixed Interest Rate	4.70%
Carrying Value of debt	27.2	28.1
Recourse Fixed Rate Debt Unsecured Fifteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	6/29/2007
Final Maturity	May 31, 2012
Fixed Interest Rate	4.25%
Carrying Value of debt	22.7	23.4
Recourse Fixed Rate Debt Unsecured Sixteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/27/2010
Final Maturity	Oct. 31, 2018
Fixed Interest Rate	3.84%
Carrying Value of debt	20.7	21.4
Recourse Fixed Rate Debt Unsecured Seventeen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	11/29/2010
Final Maturity	Nov. 30, 2018
Fixed Interest Rate	3.70%
Carrying Value of debt	20.7	21.4
Recourse Fixed Rate Debt Unsecured Eighteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	2/11/2002
Final Maturity	Jul. 31, 2012
Fixed Interest Rate	5.73%
Carrying Value of debt	0.6	1.4
Recourse Fixed Rate Debt Unsecured Nineteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	2/11/2002
Final Maturity	Jan. 31, 2012
Fixed Interest Rate	5.83%
Carrying Value of debt	0.2	1.0
Recourse Fixed Rate Debt Unsecured Twenty [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	6/22/2004
Final Maturity	Jun. 15, 2011
Fixed Interest Rate	6.27%
Carrying Value of debt	0	181.8
Recourse Fixed Rate Debt Unsecured Twenty One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	9/25/2006
Final Maturity	Aug. 31, 2011
Fixed Interest Rate	3.45%
Carrying Value of debt	0	18.8
Recourse Floating Rate Debt Unsecured Four [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	05/17/2011
Final Maturity	May 13, 2013
Floating Interest Rate	1.50%
Carrying Value of debt	100.0	0
Recourse Floating Rate Debt Unsecured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	3/18/2008
Final Maturity	Mar. 18, 2014
Floating Interest Rate	1.46%
Carrying Value of debt	150.0	150.0
Nonrecourse Fixed Rate Debt Secured Four [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	39295
Final Maturity	Jun. 30, 2017
Fixed Interest Rate	6.77%
Carrying Value of debt	2.5	2.6
Nonrecourse Fixed Rate Debt Secured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	6/13/2006
Final Maturity	Dec. 31, 2013
Fixed Interest Rate	6.26%
Carrying Value of debt	22.2	22.9
Nonrecourse Fixed Rate Debt Secured One Member
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	9/30/1997
Final Maturity	Sep. 20, 2016
Fixed Interest Rate	6.69%
Carrying Value of debt	45.2	56.2
Recourse Fixed Rate Debt Secured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	11/6/2008
Final Maturity	Nov. 15, 2013
Fixed Interest Rate	9.00%
Carrying Value of debt	168.3	179.0
Recourse Floating Rate Debt Unsecured Nine [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/27/2010
Final Maturity	Jun. 30, 2012
Floating Interest Rate	3.59%
Carrying Value of debt	5.2	5.4
Non Recourse Fixed Rate Debt Eight [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	8/1/2007
Final Maturity	Jul. 31, 2017
Fixed Interest Rate	6.78%
Carrying Value of debt	0.8	0.8
Nonrecourse Fixed Rate Debt Secured Six [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	5/11/2007
Final Maturity	May 31, 2012
Fixed Interest Rate	6.06%
Carrying Value of debt	1.4	1.5
Recourse Floating Rate Debt Unsecured Eight [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	3/1/2010
Final Maturity	Feb. 28, 2015
Floating Interest Rate	3.31%
Carrying Value of debt	13.0	13.4
Recourse Floating Rate Debt Unsecured Three [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	6/30/2006
Final Maturity	Jun. 28, 2013
Floating Interest Rate	0.93%
Carrying Value of debt	100.0	100.0
Recourse Floating Rate Debt Unsecured Seven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	09/02/2011
Final Maturity	Nov. 30, 2016
Floating Interest Rate	2.53%
Carrying Value of debt	15.6	0
Recourse Floating Rate Debt Unsecured Five [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/15/2010
Final Maturity	Oct. 31, 2015
Floating Interest Rate	2.97%
Carrying Value of debt	49.9	13.4
Nonrecourse Fixed Rate Debt Secured Three [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	5/29/2007
Final Maturity	May 31, 2012
Fixed Interest Rate	5.84%
Carrying Value of debt	3.5	3.6
Recourse Floating Rate Debt Unsecured Ten [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/31/2003
Final Maturity	Sep. 30, 2013
Floating Interest Rate	2.57%
Carrying Value of debt	4.3	6.9
Recourse Floating Rate Debt Unsecured Eleven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	11/29/2010
Final Maturity	Dec. 31, 2012
Floating Interest Rate	3.45%
Carrying Value of debt	1.9	5.4
Recourse Floating Rate Debt Unsecured Twelve [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/15/2003
Final Maturity	Dec. 15, 2012
Floating Interest Rate	2.44%
Carrying Value of debt	1.2	2.5
Recourse Floating Rate Debt Unsecured Thirteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/31/2003
Final Maturity	Dec. 31, 2012
Floating Interest Rate	2.37%
Carrying Value of debt	0.3	0.6
Nonrecourse Fixed Rate Debt Secured Five [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	6/1/2007
Final Maturity	May 31, 2012
Fixed Interest Rate	6.72%
Carrying Value of debt	2.4	2.4
Total Nonrecourse Fixed Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Carrying Value of debt	79.3	147.3
Nonrecourse Fixed Rate Debt Secured Seven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	6/16/2006
Final Maturity	Apr. 29, 2016
Fixed Interest Rate	6.80%
Carrying Value of debt	1.3	1.4
Recourse Floating Rate Debt Unsecured Six [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/06/2011
Final Maturity	Aug. 31, 2016
Floating Interest Rate	2.78%
Carrying Value of debt	29.8	0
Recourse Floating Rate Debt Unsecured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/12/2011
Final Maturity	Jun. 12, 2017
Floating Interest Rate	2.19%
Carrying Value of debt	150.0	0
Nonrecourse Fixed Rate Debt Secured Nine [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	6/28/2007
Final Maturity	Mar. 05, 2011
Fixed Interest Rate	6.34%
Carrying Value of debt	0	12.4
Nonrecourse Fixed Rate Debt Secured Ten [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	10/10/2006
Final Maturity	Sep. 30, 2011
Fixed Interest Rate	5.98%
Carrying Value of debt	0	12.1
Nonrecourse Fixed Rate Debt Secured Eleven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	6/16/2006
Final Maturity	Aug. 31, 2011
Fixed Interest Rate	5.37%
Carrying Value of debt	0	11.2
Nonrecourse Fixed Rate Debt Secured Twelve [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/21/2006
Final Maturity	Aug. 01, 2011
Fixed Interest Rate	5.89%
Carrying Value of debt	0	6.1
Nonrecourse Floating Rate Debt Secured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Various
Final Maturity	May 08, 2014
Floating Interest Rate	1.53%
Carrying Value of debt	51.0	52.5
Nonrecourse Fixed Rate Debt Secured Fourteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	6/29/2007
Final Maturity	Jul. 31, 2012
Fixed Interest Rate	6.54%
Carrying Value of debt	0	5.4
Nonrecourse Fixed Rate Debt Secured Fifteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	10/1/2007
Final Maturity	Feb. 05, 2011
Fixed Interest Rate	5.62%
Carrying Value of debt	0	4.4
Nonrecourse Fixed Rate Debt Secured Thirteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/21/2006
Final Maturity	Mar. 31, 2011
Fixed Interest Rate	5.89%
Carrying Value of debt	0	4.3
Nonrecourse Floating Rate Debt Secured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Various
Final Maturity	Jan. 15, 2015
Floating Interest Rate	1.53%
Carrying Value of debt	24.4	25.3
Recourse Floating Rate Debt Secured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	12/19/2011
Final Maturity	Dec. 19, 2020
Floating Interest Rate	2.41%
Carrying Value of debt	100.0	0
Recourse Floating Rate Debt Secured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	5/14/2009
Final Maturity	May 14, 2014
Floating Interest Rate	3.00%
Carrying Value of debt	41.4	44.8
Total Recourse Fixed Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Carrying Value of debt	2,615.9	2,448.1
Total Recourse Floating Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Carrying Value of debt	726.6	342.4
Total Nonrecourse Floating Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Carrying Value of debt	$ 75.4	$ 77.8