Debt (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Commercial Paper and Borrowings Under Bank Credit Facilities
Balance	$ 273.6		$ 28.6
Weighted average interest rate	0.72%		1.26%
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	3,281.1		3,498.2
Debt discount, net	(8.3)		(9.3)
Debt adjustment for fair value hedges	10.2		15.3
Total Debt	3,283.0		3,504.2
Recourse Fixed Rate Debt Unsecured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Apr. 30, 2009
Final Maturity	May 15, 2014
Fixed Interest Rate	8.75%
Total 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	Feb. 05, 2010
Final Maturity	May 15, 2015
Fixed Interest Rate	4.75%
Total Debt	250.0		250.0
Recourse Fixed Rate Debt Unsecured Three [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Nov. 19, 2010
Final Maturity	Jul. 15, 2016
Fixed Interest Rate	3.50%
Total 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	May 27, 2011
Final Maturity	Jun. 01, 2021
Fixed Interest Rate	4.85%
Total 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	Jun. 11, 2012
Final Maturity	Jun. 15, 2022
Fixed Interest Rate	4.75%
Total Debt	250.0		0
Recourse Fixed Rate Debt Unsecured Six [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Feb. 06, 2008
Final Maturity	Feb. 15, 2018
Fixed Interest Rate	6.00%
Total 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	Mar. 03, 2006
Final Maturity	Mar. 01, 2016
Fixed Interest Rate	5.80%
Total Debt	200.0		200.0
Recourse Fixed Rate Debt Secured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Nov. 06, 2008
Final Maturity	Nov. 15, 2013
Fixed Interest Rate	9.00%
Total Debt	159.9		168.3
Recourse Fixed Rate Debt Unsecured Eight [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 22, 2005
Final Maturity	Dec. 22, 2015
Fixed Interest Rate	5.75%
Total Debt	100.0		115.0
Recourse Fixed Rate Debt Unsecured Nine [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Sep. 20, 2011
Final Maturity	Jul. 15, 2016
Fixed Interest Rate	3.50%
Total Debt	100.0		100.0
Recourse Fixed Rate Debt Unsecured Ten [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Apr. 14, 2005
Final Maturity	Apr. 15, 2015
Fixed Interest Rate	5.70%
Total Debt	100.0		100.0
Recourse Fixed Rate Debt Unsecured Eleven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Sep. 20, 2011
Final Maturity	Jun. 01, 2021
Fixed Interest Rate	4.85%
Total Debt	50.0		50.0
Recourse Fixed Rate Debt Unsecured Twelve [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 27, 2010
Final Maturity	Oct. 31, 2018
Fixed Interest Rate	3.84%
Total Debt	21.1		20.7
Recourse Fixed Rate Debt Unsecured Thirteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Nov. 29, 2010
Final Maturity	Nov. 30, 2018
Fixed Interest Rate	3.70%
Total Debt	19.8		20.7
Recourse Fixed Rate Debt Unsecured Fourteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Sep. 24, 2009
Final Maturity	Oct. 01, 2012
Fixed Interest Rate	4.75%
Total Debt	0		300.0
Recourse Fixed Rate Debt Unsecured Fifteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Oct. 11, 2006
Final Maturity	Feb. 15, 2012
Fixed Interest Rate	5.50%
Total Debt	0		200.0
Recourse Fixed Rate Debt Unsecured Sixteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 29, 2006
Final Maturity	Dec. 11, 2012
Fixed Interest Rate	3.49%
Total Debt	0		40.5
Recourse Fixed Rate Debt Unsecured Seventeen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 18, 2007
Final Maturity	Nov. 30, 2012
Fixed Interest Rate	4.70%
Total Debt	0		27.2
Recourse Fixed Rate Debt Unsecured Eighteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jun. 29, 2007
Final Maturity	May 31, 2012
Fixed Interest Rate	4.25%
Total Debt	0		22.7
Recourse Fixed Rate Debt Unsecured Nineteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Feb. 11, 2002
Final Maturity	Jul. 31, 2012
Fixed Interest Rate	5.73%
Total Debt	0		0.6
Recourse Fixed Rate Debt Unsecured Twenty [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Feb. 11, 2002
Final Maturity	Jan. 31, 2012
Fixed Interest Rate	5.83%
Total Debt	0		0.2
Recourse Fixed Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	2,250.8		2,615.9
Recourse Floating Rate Debt Unsecured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 12, 2011	[1]
Final Maturity	Jun. 12, 2017	[1]
Floating Interest Rate	2.25%	[1]
Total Debt	107.5	[1]	115.0	[1]
Recourse Floating Rate Debt Unsecured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 18, 2008	[1]
Final Maturity	Mar. 18, 2014	[1]
Floating Interest Rate	1.25%	[1]
Total Debt	100.0	[1]	150.0	[1]
Recourse Floating Rate Debt Unsecured Three [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	May 17, 2011	[1]
Final Maturity	May 17, 2013	[1]
Floating Interest Rate	1.50%	[1]
Total Debt	100.0	[1]	100.0	[1]
Recourse Floating Rate Debt Unsecured Four [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 21, 2012	[1]
Final Maturity	Dec. 21, 2017	[1]
Floating Interest Rate	1.88%	[1]
Total Debt	100.0	[1]	0	[1]
Recourse Floating Rate Debt Secured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 19, 2011	[1]
Final Maturity	Dec. 19, 2020	[1]
Floating Interest Rate	2.10%	[1]
Total Debt	95.5	[1]	100.0	[1]
Recourse Floating Rate Debt Unsecured Five [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jun. 30, 2006	[1]
Final Maturity	Jun. 28, 2013	[1]
Floating Interest Rate	0.90%	[1]
Total Debt	70.0	[1]	100.0	[1]
Recourse Floating Rate Debt Unsecured Six [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Aug. 31, 2012	[1]
Final Maturity	Dec. 31, 2019	[1]
Floating Interest Rate	2.47%	[1]
Total Debt	66.0	[1]	0	[1]
Recourse Floating Rate Debt Unsecured Seven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 15, 2010	[1]
Final Maturity	Oct. 31, 2015	[1]
Floating Interest Rate	1.58%	[1]
Total Debt	42.9	[1]	49.9	[1]
Recourse Floating Rate Debt Unsecured Eight [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 29, 2006	[1]
Final Maturity	Sep. 30, 2016	[1]
Floating Interest Rate	2.09%	[1]
Total Debt	41.2	[1]	0	[1]
Recourse Floating Rate Debt Secured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	May 14, 2009	[1]
Final Maturity	May 14, 2014	[1]
Floating Interest Rate	2.86%	[1]
Total Debt	38.0	[1]	41.4	[1]
Recourse Floating Rate Debt Unsecured Nine [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jan. 12, 2012	[1]
Final Maturity	Jun. 12, 2017	[1]
Floating Interest Rate	2.25%	[1]
Total Debt	35.0	[1]	0	[1]
Recourse Floating Rate Debt Unsecured Ten [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 18, 2007	[1]
Final Maturity	Oct. 31, 2016	[1]
Floating Interest Rate	2.01%	[1]
Total Debt	27.7	[1]	0	[1]
Recourse Floating Rate Debt Unsecured Eleven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 06, 2011	[1]
Final Maturity	Aug. 31, 2016	[1]
Floating Interest Rate	1.56%	[1]
Total Debt	25.8	[1]	29.8	[1]
Recourse Floating Rate Debt Unsecured Twelve [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jun. 29, 2007	[1]
Final Maturity	Sep. 30, 2016	[1]
Floating Interest Rate	2.17%	[1]
Total Debt	18.4	[1]	0	[1]
Recourse Floating Rate Debt Unsecured Thirteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Sep. 02, 2011	[1]
Final Maturity	Nov. 30, 2016	[1]
Floating Interest Rate	1.24%	[1]
Total Debt	13.4	[1]	15.6	[1]
Recourse Floating Rate Debt Unsecured Fourteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 01, 2010	[1]
Final Maturity	Feb. 28, 2015	[1]
Floating Interest Rate	1.97%	[1]
Total Debt	13.2	[1]	13.0	[1]
Recourse Floating Rate Debt Unsecured Fifteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 31, 2003	[1]
Final Maturity	Sep. 30, 2013	[1]
Floating Interest Rate	1.73%	[1]
Total Debt	1.8	[1]	4.3	[1]
Recourse Floating Rate Debt Unsecured Sixteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 27, 2010	[1]
Final Maturity	Jun. 30, 2012	[1]
Floating Interest Rate	2.84%	[1]
Total Debt	0	[1]	5.2	[1]
Recourse Floating Rate Debt Unsecured Seventeen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Nov. 29, 2010	[1]
Final Maturity	Dec. 31, 2012	[1]
Floating Interest Rate	2.20%	[1]
Total Debt	0	[1]	1.9	[1]
Recourse Floating Rate Debt Unsecured Eighteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 15, 2003	[1]
Final Maturity	Dec. 15, 2012	[1]
Floating Interest Rate	1.63%	[1]
Total Debt	0	[1]	1.2	[1]
Recourse Floating Rate Debt Unsecured Nineteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 31, 2003	[1]
Final Maturity	Dec. 31, 2012	[1]
Floating Interest Rate	1.53%	[1]
Total Debt	0	[1]	0.3	[1]
Total Recourse Floating Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	896.4	[1]	727.6	[1]
Nonrecourse Fixed Rate Debt Secured One Member
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Sep. 30, 1997
Final Maturity	Sep. 20, 2016
Fixed Interest Rate	6.69%
Total Debt	35.1		45.2
Nonrecourse Fixed Rate Debt Secured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jun. 13, 2006
Final Maturity	Dec. 31, 2013
Fixed Interest Rate	6.26%
Total Debt	21.4		22.2
Nonrecourse Fixed Rate Debt Secured Three [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Aug. 01, 2007
Final Maturity	Jun. 30, 2017
Fixed Interest Rate	6.77%
Total Debt	2.5		2.5
Nonrecourse Fixed Rate Debt Secured Four [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jun. 16, 2006
Final Maturity	Apr. 29, 2016
Fixed Interest Rate	6.80%
Total Debt	1.3		1.3
Nonrecourse Fixed Rate Debt Secured Five [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Aug. 01, 2007
Final Maturity	Jul. 31, 2017
Fixed Interest Rate	6.78%
Total 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	May 29, 2007
Final Maturity	May 31, 2012
Fixed Interest Rate	5.84%
Total Debt	0		3.5
Nonrecourse Fixed Rate Debt Secured Seven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jun. 01, 2007
Final Maturity	May 31, 2012
Fixed Interest Rate	6.27%
Total Debt	0		2.4
Nonrecourse Fixed Rate Debt Secured Eight [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	May 11, 2007
Final Maturity	May 31, 2012
Fixed Interest Rate	6.06%
Total Debt	0		1.4
Total Nonrecourse Fixed Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	61.1		79.3
Nonrecourse Floating Rate Debt Secured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Final Maturity	May 08, 2014	[1]
Floating Interest Rate	1.46%	[1]
Total Debt	49.3	[1]	51.0	[1]
Nonrecourse Floating Rate Debt Secured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Final Maturity	Jan. 15, 2015	[1]
Floating Interest Rate	1.46%	[1]
Total Debt	23.5	[1]	24.4	[1]
Total Nonrecourse Floating Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	$ 72.8	[1]	$ 75.4	[1]

[1] Floating rates as of December 31, 2012.