Senior Notes - Additional Information (Details)	Sep. 30, 2020 USD ($)	Dec. 06, 2019 USD ($)	Sep. 30, 2019 USD ($)	May 08, 2018 USD ($)	Jul. 14, 2010 USD ($) Contract
Debt Disclosure [Line Items]
Debt issuance costs	$ 10,600,000		$ 5,200,000
July 2010 Senior Notes
Debt Disclosure [Line Items]
Senior Notes issued in a private placement to a group of institutional investors					$ 245,000,000
Number of series of Senior Notes issued | Contract					4
May 2018 Senior Notes
Debt Disclosure [Line Items]
Senior Notes issued in a private placement to a group of institutional investors				$ 400,000,000
December 2019 Senior Notes
Debt Disclosure [Line Items]
Senior Notes issued in a private placement to a group of institutional investors		$ 350,000,000