Notes Payable - Note Repayment and Note Issuances (Details) £ in Thousands	1 Months Ended				3 Months Ended		9 Months Ended
	Jul. 31, 2020 USD ($)	May 31, 2020 USD ($)	Jan. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019	May 31, 2019 GBP (£)
Debt
Principal payments on notes payable							$ 250,000,000	$ 0
Loss on extinguishment of debt					$ 0	$ 0	9,819,000	0
Proceeds issuance of notes							972,766,000	$ 895,774,000
3.250% notes, issued in July 2020 and due in January 2031
Debt
Face amount of notes	$ 350,000,000
3.250% notes, issued in May 2020 and due in January 2031
Debt
Face amount of notes		$ 600,000,000
Notes payable | 5.750% notes, issued in June 2010 and due in January 2021
Debt
Principal payments on notes payable			$ 250,000,000.0
Interest rate (as a percent)			5.75%						5.75%
Loss on extinguishment of debt							$ 9,800,000
Notes payable | 3.250% notes, issued in July 2020 and due in January 2031
Debt
Face amount of notes	$ 350,000,000.0
Interest rate (as a percent)	3.25%
Price of par value (as a percent)	108.241%
Effective yield (as a percent)	2.341%
Proceeds issuance of notes	$ 378,800,000
Notes payable | 3.250% notes, issued in May 2020 and due in January 2031
Debt
Face amount of notes		$ 600,000,000.0
Price of par value (as a percent)		98.987%
Effective yield (as a percent)		3.364%
Proceeds issuance of notes		$ 593,900,000
Notes payable | 3.250% notes, issued in June 2019 and due in June 2029
Debt
Face amount of notes				$ 500,000,000.0
Interest rate (as a percent)				3.25%	3.25%		3.25%
Price of par value (as a percent)				99.359%
Effective yield (as a percent)				3.326%
Proceeds issuance of notes				$ 496,800,000
Notes payable | 2.730% notes, issued in May 2019 and due in May 2034
Debt
Face amount of notes | £										£ 315,000
Interest rate (as a percent)					2.73%		2.73%			2.73%