Borrowings - Summary of Outstanding Borrowings (Details) - USD ($)	Jun. 30, 2018	Jun. 11, 2018	Dec. 31, 2017
Borrowings
Total borrowings at par value	$ 3,349,900,000		$ 3,049,900,000
Fair value hedge accounting adjustments, net	(900,000)		500,000
Debt issuance costs and unamortized discount, net	(17,200,000)		(16,800,000)
Total borrowings at carrying value	$ 3,331,800,000		3,033,600,000
Weighted-average effective interest rate	4.70%
3.650% notes (effective rate of 5.4%) due 2018
Borrowings
Total borrowings at par value	$ 400,000,000		$ 400,000,000
Stated interest rate	3.65%		3.65%
Effective interest rate	5.40%
3.350% notes due 2019
Borrowings
Total borrowings at par value	$ 250,000,000		$ 250,000,000
Stated interest rate	3.35%		3.35%
Floating rate notes (effective rate of 3.4%) due 2019
Borrowings
Total borrowings at par value	$ 250,000,000		$ 250,000,000
Effective interest rate	3.40%
5.253% notes due 2020
Borrowings
Total borrowings at par value	$ 324,900,000		$ 324,900,000
Stated interest rate	5.253%		5.253%
3.600% notes due 2022
Borrowings
Total borrowings at par value	$ 500,000,000		$ 500,000,000
Stated interest rate	3.60%		3.60%
4.250% notes (effective rate of 4.5%) due 2023
Borrowings
Total borrowings at par value		$ 300,000,000	$ 0
Stated interest rate	4.25%	4.25%	0.00%
Effective interest rate	4.50%
6.200% notes due 2036
Borrowings
Total borrowings at par value	$ 500,000,000		$ 500,000,000
Stated interest rate	6.20%		6.20%
6.200% notes due 2040
Borrowings
Total borrowings at par value	$ 250,000,000		$ 250,000,000
Stated interest rate	6.20%		6.20%
Term loan facility borrowings (effective rate of 3.6%)
Borrowings
Total borrowings at par value	$ 575,000,000		$ 575,000,000
Effective interest rate	3.60%