Debt (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Commercial Paper and Borrowings Under Bank Credit Facilities
Balance	$ 72.1	$ 23.6
Weighted average interest rate	0.56%	0.80%
Outstanding balances of debt obligations and the applicable interest rates
Total debt principal	4,203.8	3,842.0
Debt discount, net	(8.6)	(8.9)
Debt adjustment for fair value hedges	0.6	5.4
Total Debt	4,195.8	3,838.5
Recourse Fixed Rate Debt Unsecured One [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 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	Dec. 22, 2005
Final Maturity	Dec. 22, 2015
Fixed Interest Rate	5.75%
Total Debt	70.0	85.0
Recourse Fixed Rate Debt Unsecured Four [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	250.0	200.0
Recourse Fixed Rate Debt Unsecured Five [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	200.0	250.0
Recourse Fixed Rate Debt Unsecured Six [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 Seven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 04, 2014
Final Maturity	Mar. 04, 2017
Fixed Interest Rate	1.25%
Total Debt	300.0	0
Recourse Fixed Rate Debt Unsecured Eight [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 Nine [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 19, 2013
Final Maturity	Jul. 30, 2018
Fixed Interest Rate	2.38%
Total Debt	250.0	250.0
Recourse Fixed Rate Debt Unsecured Ten [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	15.7	22.0
Recourse Fixed Rate Debt Unsecured Eleven [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	12.1	17.2
Recourse Fixed Rate Debt Unsecured Twelve [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Nov. 19, 2013
Final Maturity	Mar. 15, 2019
Fixed Interest Rate	2.50%
Total Debt	300.0	300.0
Recourse Fixed Rate Debt Unsecured Thirteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 04, 2014
Final Maturity	Jul. 30, 2019
Fixed Interest Rate	2.50%
Total Debt	250.0	0
Recourse Fixed Rate Debt Unsecured Fourteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Oct. 31, 2014
Final Maturity	Mar. 30, 2020
Fixed Interest Rate	2.60%
Total Debt	250.0	0
Recourse Fixed Rate Debt Unsecured Fifteen [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 Sixteen [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 Seventeen [Member] [Domain]
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	250.0
Recourse Fixed Rate Debt Unsecured Eighteen [Member] [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 19, 2013
Final Maturity	Mar. 30, 2023
Fixed Interest Rate	3.90%
Total Debt	250.0	250.0
Recourse Fixed Rate Debt Unsecured Nineteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 04, 2014
Final Maturity	Mar. 15, 2044
Fixed Interest Rate	5.20%
Total Debt	300.0	0
Recourse Fixed Rate Debt Unsecured Twenty [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	0	300.0
Recourse Fixed Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	3,647.8	2,874.2
Recourse Floating Rate Debt Unsecured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 15, 2010
Final Maturity	Oct. 31, 2015
Floating Interest Rate	1.47%
Total Debt	24.8	36.4
Recourse Floating Rate Debt Unsecured Three [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Sep. 02, 2011
Final Maturity	Aug. 31, 2016
Floating Interest Rate	1.13%
Total Debt	8.0	11.5
Recourse Floating Rate Debt Unsecured Four [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 29, 2006
Final Maturity	Sep. 30, 2016
Floating Interest Rate	2.08%
Total Debt	18.9	32.2
Recourse Floating Rate Debt Secured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 19, 2011
Final Maturity	Dec. 19, 2020
Floating Interest Rate	2.10%
Total debt principal	0	91.1
Recourse Floating Rate Debt Unsecured Five [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jun. 29, 2007
Final Maturity	Sep. 30, 2016
Floating Interest Rate	2.03%
Total Debt	8.5	14.4
Recourse Floating Rate Debt Unsecured Six [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 18, 2007
Final Maturity	Oct. 31, 2016
Floating Interest Rate	1.98%
Total Debt	12.7	21.6
Recourse Floating Rate Debt Unsecured Seven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 21, 2012
Final Maturity	Dec. 21, 2017
Floating Interest Rate	1.39%
Total Debt	100.0	100.0
Recourse Floating Rate Debt Unsecured Eight [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jan. 22, 2013
Final Maturity	Dec. 21, 2017
Floating Interest Rate	1.39%
Total Debt	10.0	70.0
Recourse Floating Rate Debt Secured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 06, 2011
Final Maturity	Aug. 31, 2016
Floating Interest Rate	1.57%
Total Debt	15.3	22.1
Recourse Floating Rate Debt Unsecured Nine [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Aug. 31, 2012
Final Maturity	Dec. 31, 2019
Floating Interest Rate	2.33%
Total Debt	60.5	68.7
Recourse Floating Rate Debt Unsecured Ten [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jun. 27, 2013
Final Maturity	Dec. 31, 2020
Floating Interest Rate	1.87%
Total debt principal	121.0	137.5
Recourse Floating Rate Debt Un Secured Eleven [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Oct. 30, 2013
Final Maturity	Dec. 31, 2020
Floating Interest Rate	1.91%
Total debt principal	30.2	34.4
Recourse Floating Rate Debt Un Secured Twelve [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	May 05, 2014
Final Maturity	Dec. 31, 2020
Floating Interest Rate	1.99%
Total debt principal	30.2	0
Recourse Floating Rate Debt Un Secured Thirteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Aug. 28, 2014
Final Maturity	Aug. 28, 2024
Floating Interest Rate	1.54%
Total debt principal	100.0	0
Recourse Floating Rate Debt Unsecured Fourteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 01, 2010
Final Maturity	Feb. 28, 2015
Floating Interest Rate	1.83%
Total debt principal	0	13.7
Recourse Floating Rate Debt Unsecured Fifteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 12, 2011
Final Maturity	Jun. 12, 2017
Floating Interest Rate	1.63%
Total debt principal	0	103.5
Recourse Floating Rate Debt Unsecured Sixteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Aug. 12, 2013
Final Maturity	Jun. 12, 2017
Floating Interest Rate	1.63%
Total debt principal	0	71.1
Recourse Floating Rate Debt Unsecured Seventeen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jan. 12, 2012
Final Maturity	Jun. 12, 2017
Floating Interest Rate	1.63%
Total debt principal	0	31.5
Recourse Floating Rate Debt Unsecured Eighteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Feb. 21, 2013
Final Maturity	Dec. 21, 2017
Floating Interest Rate	1.81%
Total debt principal	0	35.0
Total Recourse Floating Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total debt principal	540.1	894.7
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
Floating Interest Rate	6.69%
Total debt principal	15.9	25.4
Total Nonrecourse Fixed Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total debt principal	15.9	25.4
Nonrecourse Floating Rate Debt Secured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Final Maturity	May 08, 2014
Floating Interest Rate	1.40%
Total debt principal	$ 0	$ 47.7