Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Commercial Paper and Borrowings Under Bank Credit Facilities
Balance	$ 7.4	$ 72.1
Weighted average interest rate	1.22%	0.56%
Outstanding balances of debt obligations and the applicable interest rates
Total debt principal	$ 4,205.4	$ 4,203.8
Debt discount, net	(28.5)	(26.2)
Debt adjustment for fair value hedges	1.5	0.6
Total Debt	$ 4,178.4	4,178.2
Recourse Fixed Rate Debt Unsecured One [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 Unsecured Two [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 Three [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 Four [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	300.0
Recourse Fixed Rate Debt Unsecured Five [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 Six [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 Seven [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	$ 10.9	15.7
Recourse Fixed Rate Debt Unsecured Eight [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	$ 8.1	12.1
Recourse Fixed Rate Debt Unsecured Nine [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jan. 30, 2015
Final Maturity	Dec. 31, 2018
Fixed Interest Rate	1.20%
Total Debt	$ 54.3	0.0
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	250.0
Recourse Fixed Rate Debt Unsecured Twelve [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	250.0
Recourse Fixed Rate Debt Unsecured Fourteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Feb. 06, 2015
Final Maturity	Mar. 30, 2020
Fixed Interest Rate	2.60%
Total Debt	$ 100.0	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 Seventeen [Member] [Domain]
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 Eighteen [Member] [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Feb. 06, 2015
Final Maturity	Mar. 30, 2025
Fixed Interest Rate	3.25%
Total Debt	$ 300.0	0.0
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	300.0
Recourse Fixed Rate Debt Unsecured Twenty [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Feb. 06, 2015
Final Maturity	Mar. 30, 2045
Fixed Interest Rate	4.50%
Total Debt	$ 250.0	0.0
Recourse Fixed Rate Debt Unsecured Twenty 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	$ 0.0	100.0
Recourse Fixed Rate Debt Unsecured Twenty 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	$ 0.0	250.0
Recourse Fixed Rate Debt Unsecured Twenty 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	$ 0.0	70.0
Recourse Fixed Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	$ 3,923.3	3,647.8
Recourse Floating Rate Debt Unsecured One [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	0.94%
Total Debt	$ 5.2	8.0
Recourse Floating Rate Debt Secured Two [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	1.64%
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	1.64%
Total Debt	$ 10.0	10.0
Recourse Floating Rate Debt Unsecured Four [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	1.71%
Total debt principal	$ 100.0	100.0
Recourse Floating Rate Debt Unsecured Five [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Sep. 23, 2015
Final Maturity	Sep. 23, 2025
Floating Interest Rate	1.84%
Total Debt	$ 60.0	0.0
Recourse Floating Rate Debt Unsecured Six [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	1.43%
Total Debt		24.8
Recourse Floating Rate Debt Unsecured Seven [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	1.44%
Total Debt		15.3
Recourse Floating Rate Debt Unsecured Eight [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	2.02%
Total Debt		18.9
Recourse Floating Rate Debt Unsecured Nine [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
Recourse Floating Rate Debt Unsecured Ten [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
Recourse Floating Rate Debt Un Secured Eleven [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
Recourse Floating Rate Debt Un Secured Twelve [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	1.73%
Total Debt		121.0
Recourse Floating Rate Debt Un Secured Thirteen [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	1.78%
Total Debt		30.2
Recourse Floating Rate Debt Unsecured Fourteen [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	1.85%
Total Debt		30.2
Total Recourse Floating Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	$ 275.2	540.1
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
Floating Interest Rate	6.69%
Total debt principal	$ 6.9	15.9
Total Nonrecourse Fixed Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total debt principal	6.9	$ 15.9
Line of Credit US [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 575.0