Earnings Per Share - Antidilutive Securities (Details) - $ / shares	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Feb. 18, 2020
Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Exchangeable notes, exchange price per share (in USD per share)	$ 40.87								$ 40.87			$ 40.87
Warrant exercise price (in USD per share)	$ 62.88								$ 62.88
Restricted Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)	94,577	193,709	215,564	232,828	271,881	285,433	277,758	295,405	204,969	282,801	299,412
Class B Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)		12,404,368	12,769,568	12,921,637	12,921,637	16,184,026	17,112,164	17,213,806		15,856,855
Out-of-the-money Options | Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)	1,297,500	1,498,000	959,000	689,500	446,000	443,000	30,500	754,750	1,327,500	626,500
Stock options | Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (shares)	1,225,697	1,127,509		976,594	1,131,760	1,188,987	1,012,916		1,179,538	1,009,858