Summary of Significant Accounting Policies - Income Tax and Common Stock (Details) $ in Thousands		3 Months Ended		9 Months Ended	12 Months Ended
	Jan. 01, 2018 USD ($)	Sep. 30, 2018 USD ($) vote	Dec. 31, 2017 USD ($)	Jun. 30, 2018	Sep. 30, 2018 USD ($) vote	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
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
Reclassification of stranded tax effects resulting from the Tax Cuts and Jobs Act					0
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			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