Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Commercial Paper and Borrowings Under Bank Credit Facilities
Balance	$ 15.8	$ 110.8
Weighted average interest rate	0.65%	2.77%
Outstanding balances of debt obligations and the applicable interest rates
Total debt principal	$ (4,813.9)	$ (4,473.1)
Debt discount, net	(34.9)	(35.9)
Debt adjustment for fair value hedges	1.4	(7.5)
Total Debt	$ 4,780.4	4,429.7
Recourse Fixed Rate Debt Secured One [Member] [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 Floating Rate Debt Unsecured Three [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	4.00%
Total Debt	$ 0.0	$ 100.0
Recourse Fixed Rate Debt Unsecured Four [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	100.0
Recourse Fixed Rate Debt Unsecured Three [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 Six [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 Seven [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	250.0
Recourse Fixed Rate Debt Unsecured Fifteen [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 Eight [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Nov. 05, 2018
Final Maturity	Feb. 15, 2024
Fixed Interest Rate	4.35%
Total Debt	$ 300.0	300.0
Recourse Fixed Rate Debt Unsecured Eight [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Nov. 12, 2019
Final Maturity	Nov. 05, 2024
Fixed Interest Rate	0.96%
Total Debt	$ 112.1	0.0
Recourse Fixed Rate Debt Unsecured Sixteen [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	300.0
Recourse Fixed Rate Debt Unsecured Sixteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Sep. 13, 2016
Final Maturity	Sep. 15, 2026
Fixed Interest Rate	3.25%
Total Debt	$ 350.0	350.0
Recourse Fixed Rate Debt Unsecured Sixteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Nov. 04, 2019
Final Maturity	Nov. 04, 2026
Fixed Interest Rate	1.07%
Total Debt	$ 84.1	0.0
Recourse Fixed Rate Debt Unsecured Seventeen [Member] [Domain]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Feb. 09, 2017
Final Maturity	Mar. 30, 2027
Fixed Interest Rate	3.85%
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	Nov. 02, 2017
Final Maturity	Mar. 15, 2028
Fixed Interest Rate	3.50%
Total Debt	$ 300.0	300.0
Recourse Fixed Rate Debt Unsecured Eighteen [Member] [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	May 07, 2018
Final Maturity	Nov. 07, 2028
Fixed Interest Rate	4.55%
Total Debt	$ 300.0	300.0
Recourse Fixed Rate Debt Unsecured Fifteen [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Jan. 31, 2019
Final Maturity	Apr. 01, 2029
Fixed Interest Rate	4.70%
Total Debt	$ 500.0	0.0
Recourse Fixed Rate Debt Unsecured Twenty [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 Seventeen [Member] [Domain]
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	250.0
Recourse Fixed Rate Debt Unsecured Eighteen [Member] [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	May 16, 2016
Final Maturity	May 30, 2066
Fixed Interest Rate	5.63%
Total Debt	$ 150.0	150.0
Recourse Fixed Rate Debt Unsecured Twenty [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	$ 0.0	250.0
Recourse Fixed Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	$ 4,396.2	$ 3,950.0
Recourse Floating Rate Debt Unsecured One [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Nov. 06, 2017
Final Maturity		Nov. 05, 2021
Floating Interest Rate	2.61%
Total debt principal	$ (300.0)	$ (300.0)
Recourse Floating Rate Debt Secured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 22, 2016
Final Maturity		May 24, 2024
Floating Interest Rate	0.90%
Total debt principal	$ (117.7)	$ (63.1)
Recourse Floating Rate Debt Unsecured Four [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	4.07%
Total Debt	$ 0.0	$ 60.0
Total Recourse Floating Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	417.7	523.1
Line of Credit US [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	600.0	$ 600.0
Line of Credit Facility, Capacity Available for Trade Purchases	$ 600.0