New Accounting Standards - Narrative (Details) - USD ($) $ in Thousands		Jun. 30, 2020	Mar. 31, 2020	Oct. 01, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset		$ 23,732
Operating lease, liability		24,587
Cumulative effect adjustment		1,160,644	$ 1,153,464		$ 1,900,249	$ 1,881,128	$ 1,852,394	$ 1,840,551
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment		$ (68,305)	$ (73,705)		657,475	$ 624,242	$ 614,540	$ 553,014
Cumulative effect adjustment related to ASU adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment	[1]				(182)
Cumulative effect adjustment related to ASU adoption | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment	[1]				$ (182)
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset				$ 19,900
Operating lease, liability				20,900
ASU 2016-02 | Cumulative effect adjustment related to ASU adoption | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment				$ 200

[1] Related to the Company's adoption of ASU 2016-02 and subsequent related ASUs on October 1, 2019. See Note 2, "New Accounting Pronouncements," for additional information.