Stockholders' Equity, Voting Rights, Stock Award and Incentive Plan (Details) $ / shares in Units, $ in Thousands				3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 15, 2021 $ / shares shares	Mar. 05, 2020 $ / shares shares	Jan. 02, 2020 USD ($) $ / shares	Sep. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) VoteperShare $ / shares shares	Sep. 30, 2020 USD ($)	Dec. 31, 2016 $ / shares	Dec. 31, 2020 USD ($) $ / shares shares	Jun. 05, 2020 shares	Jun. 04, 2020 shares
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Compensation payable | $				$ 47,767		$ 47,767			$ 21,543
Deferred Compensation Arrangements [Abstract]
Payments for compensation agreement | $			$ 43,700
Reduction of RSU expense due to cap and waiver of receipt of deferred compensation expense | $			$ 32,300
Deferred compensation agreement, share price (in dollars per share) | $ / shares			$ 18.8812					$ 32.8187
Stock Options [Member]
Stock Award and Incentive Plan [Abstract]
Stock option outstanding (in shares)				10,000		10,000
Stock option outstanding, exercise price (in dollars per share) | $ / shares				$ 25.55		$ 25.55
Stock Options [Member] | Maximum [Member]
Stock Award and Incentive Plan [Abstract]
Term of nonqualified stock options						10 years
RSAs [Member]
Stock Award and Incentive Plan [Abstract]
RSAs granted (in shares)		392,700
Grant date fair value (in dollars per share) | $ / shares		$ 14.31
RSA shares outstanding (in shares)				909,215		909,215			1,079,650
Average weighted grant price (in dollars per share) | $ / shares				$ 18.79		$ 18.79			$ 19.45
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Stock-based compensation expense | $				$ 900	$ 1,000	$ 3,000	$ 3,100
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]
Compensation cost related to non-vested options not yet recognized | $				$ 8,000		$ 8,000
Phantom RSAs [Member]
Stock Award and Incentive Plan [Abstract]
RSAs granted (in shares)	396,800
Grant date fair value (in dollars per share) | $ / shares	$ 25.02
RSA shares outstanding (in shares)				393,300		393,300
Average weighted grant price (in dollars per share) | $ / shares				$ 25.02		$ 25.02
Actual and projected stock based compensation expense for RSA shares and options [Abstract]
Stock-based compensation expense | $				$ 600		$ 700
Compensation payable | $				700		700
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]
Compensation cost related to non-vested options not yet recognized | $				$ 8,400		$ 8,400
Class A [Member]
Voting Rights [Abstract]
Number of votes per share | VoteperShare						1
Authorized Shares [Abstract]
Number of shares authorized (in shares)				100,000,000		100,000,000			100,000,000
Class A [Member] | Maximum [Member]
Stock Award and Incentive Plan [Abstract]
Number of shares reserved for issuance under each plan (in shares)				7,500,000		7,500,000
Class A [Member] | RSAs [Member]
Stock Award and Incentive Plan [Abstract]
Number of shares of common stock to which the grantee is entitled (in shares)	1
Class A [Member] | Phantom RSAs [Member]
Stock Award and Incentive Plan [Abstract]
Number of shares of common stock the cash value of which the grantee is entitled to (in shares)	1
Class B [Member]
Voting Rights [Abstract]
Number of votes per share | VoteperShare						10
Authorized Shares [Abstract]
Number of shares authorized (in shares)				25,000,000		25,000,000			25,000,000	25,000,000	100,000,000