Borrowings - Outstanding Borrowings (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Outstanding Borrowings
Total borrowings at par value	$ 3,085.0	$ 3,245.0
Debt issuance costs and unamortized discount, net	(14.9)	(15.7)
Total borrowings at carrying value	$ 3,070.1	3,229.3
Weighted-average effective interest rate (as a percent)	3.90%
Commercial paper
Outstanding Borrowings
Total borrowings at par value	$ 85.0	245.0
Maximum borrowing capacity	$ 1,500.0
Maximum days to maturity	397 days
Weighted-average effective interest rate (as a percent)	3.50%
Weighted-average term	1 day
3.600% notes due 2022
Outstanding Borrowings
Total borrowings at par value	$ 500.0	$ 500.0
Stated interest rate (as a percent)	3.60%	3.60%
4.250% notes due 2023
Outstanding Borrowings
Total borrowings at par value	$ 300.0	$ 300.0
Stated interest rate (as a percent)	4.25%	4.25%
2.850% notes due 2025
Outstanding Borrowings
Total borrowings at par value	$ 500.0	$ 500.0
Stated interest rate (as a percent)	2.85%	2.85%
6.200% notes due 2036
Outstanding Borrowings
Total borrowings at par value	$ 500.0	$ 500.0
Stated interest rate (as a percent)	6.20%	6.20%
6.200% notes due 2040
Outstanding Borrowings
Total borrowings at par value	$ 250.0	$ 250.0
Stated interest rate (as a percent)	6.20%	6.20%
Term loan facility borrowing (effective rate of 3.0%)
Outstanding Borrowings
Total borrowings at par value	$ 950.0	$ 950.0
Effective interest rate (as a percent)	2.30%