Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Commercial Paper and Borrowings Under Bank Credit Facilities
Balance	$ 4.3	$ 3.8
Weighted average interest rate	1.02%	1.02%
Outstanding balances of debt obligations and the applicable interest rates
Total debt principal	$ 4,412.6	$ 4,286.2
Debt discount, net	(36.2)	(33.6)
Debt adjustment for fair value hedges	(4.7)	0.6
Total Debt	$ 4,371.7	4,253.2
Recourse Fixed Rate Debt Unsecured Three [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Aug. 28, 1996
Final Maturity	Feb. 28, 2018
Fixed Interest Rate	7.86%
Total Debt	$ 11.6	0.0
Recourse Fixed Rate Debt Unsecured Four [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 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	2.78%
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	Dec. 27, 2010
Final Maturity	Oct. 31, 2018
Fixed Interest Rate	3.84%
Total Debt	$ 12.0	10.5
Recourse Fixed Rate Debt Unsecured Five [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	$ 3.0	5.3
Recourse Fixed Rate Debt Unsecured Six [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	$ 60.0	52.6
Recourse Fixed Rate Debt Unsecured Seven [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 Eight [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 Nine [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 Twelve [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 Thirteen [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 Twelve [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 Fourteen [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 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	Feb. 09, 2017
Final Maturity	Mar. 30, 2027
Fixed Interest Rate	3.85%
Total Debt	$ 300.0	0.0
Recourse Fixed Rate Debt Unsecured Seventeen [Member] [Domain]
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	0.0
Recourse Fixed Rate Debt Unsecured Eighteen [Member] [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 Nineteen [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	250.0
Recourse Fixed Rate Debt Unsecured Twenty [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 One [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	$ 0.0	200.0
Recourse Fixed Rate Debt Unsecured Twenty Two [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	$ 0.0	300.0
Recourse Fixed Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	$ 3,986.6	3,868.4
Recourse Floating Rate Debt Unsecured Five [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	2.11%
Total debt principal	$ 200.0	0.0
Recourse Floating Rate Debt Secured Two [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Dec. 20, 2016
Final Maturity	Dec. 20, 2021
Floating Interest Rate	0.85%
Total debt principal	$ 66.0	57.8
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	2.93%
Total Debt	$ 60.0	60.0
Recourse Floating Rate Debt Unsecured Five [Member]
Outstanding balances of debt obligations and the applicable interest rates
Date of Issue	Mar. 02, 2016
Final Maturity	Mar. 02, 2021
Floating Interest Rate	2.57%
Total Debt	$ 0.0	200.0
Total Recourse Floating Rate Debt [Member]
Outstanding balances of debt obligations and the applicable interest rates
Total Debt	426.0	$ 417.8
Line of Credit US [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 600.0