Notes Payable - Note Issuances (Details)	1 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Apr. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	May 31, 2019 GBP (£)	Dec. 31, 2018
Debt
Proceeds from notes payable			$ 895,774,000	$ 497,500,000
Notes payable | 2.730% notes, issued in May 2019 and due in May 2034
Debt
Face amount of notes | £					£ 315,000,000
Stated interest rate (as a percent)	2.73%		2.73%		2.73%
Notes payable | 3.250% notes, issued in June 2019 and due in June 2029
Debt
Face amount of notes	$ 500,000,000		$ 500,000,000
Stated interest rate (as a percent)	3.25%		3.25%
Offering price (percent)	99.36%		99.36%
Effective yield (as a percent)	3.326%		3.326%
Proceeds from notes payable	$ 492,200,000
Notes payable | 3.875% notes, issued in April 2018 and due in April 2025
Debt
Face amount of notes		$ 500,000,000
Stated interest rate (as a percent)	3.875%	3.875%	3.875%			3.875%
Offering price (percent)		99.50%
Effective yield (as a percent)		3.957%
Proceeds from notes payable		$ 493,100,000