Summary of Significant Accounting Policies - Income Tax and Common Stock (Details) $ in Thousands		3 Months Ended		12 Months Ended
	Jan. 01, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019 USD ($) vote	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Class of Stock [Line Items]
Blended tax rate					24.50%
Charge resulting from remeasured deferred tax asset		$ 1,900	$ 2,800		$ 2,100
Charge resulting from recording valuation allowance not likely to be utilized					2,600
Unrecognized tax benefits, interest and penalties				$ 200	200	$ 200
Common Stock Class B Voting
Class of Stock [Line Items]
Number of votes per share | vote				1
Retained Earnings
Class of Stock [Line Items]
Reclassification of stranded tax effects resulting from the Tax Cuts and Jobs Act					$ 1,455
Retained Earnings | Accounting Standards Update 2018-02
Class of Stock [Line Items]
Reclassification of stranded tax effects resulting from the Tax Cuts and Jobs Act	$ 1,500