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Subsequent Events (Details) $ / shares in Units, $ in Thousands													2 Months Ended	3 Months Ended									6 Months Ended	12 Months Ended
	Mar. 05, 2020 shares	Feb. 04, 2020 $ / shares	Jan. 02, 2020 USD ($) $ / shares	Sep. 16, 2019 $ / shares	Jun. 30, 2019 shares	Apr. 16, 2019 $ / shares	Apr. 01, 2019 USD ($) $ / shares	Sep. 17, 2018 shares	Aug. 07, 2018 shares	Jul. 02, 2018 USD ($) $ / shares	Apr. 04, 2018 shares	Jan. 05, 2018 shares	Mar. 06, 2020 $ / shares shares	Dec. 31, 2019 $ / ft² $ / shares shares	Sep. 30, 2019 $ / shares	Jun. 30, 2019 $ / shares	Mar. 31, 2019 $ / shares	Dec. 31, 2018 $ / shares shares	Sep. 30, 2018 $ / shares	Jun. 30, 2018 $ / shares	Mar. 31, 2018 $ / shares	Sep. 30, 2017 $ / shares	Jun. 30, 2018 $ / shares	Dec. 31, 2019 USD ($) $ / ft² $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2017	Dec. 27, 2017	Nov. 30, 2015
Subsequent Event [Line Items]
Compensation record expenses due to accelerated vesting																									$ 200
Payments for compensation agreement							$ 11,000			$ 28,300
Reduction of RSU expense due to cap and DCCA being indexed to GBL stock price							$ 4,500			$ 2,600
Deferred compensation agreement, share price (in dollars per share) \| $ / shares							$ 20.7916			$ 27.1837												$ 29.1875	$ 29.6596
Dividends declared (in dollars per share) \| $ / shares														$ 0.02	$ 0.02	$ 0.02	$ 0.02	$ 0.02	$ 0.02	$ 0.02	$ 0.02			$ 0.08	$ 0.08
Average price per share of repurchased shares (in dollars per share) \| $ / shares				$ 20.07		$ 21.00
Base rent per square foot (in dollars per square foot) \| $ / ft²														18										18
Common stock, shares outstanding (in shares) \| shares														27.4				29.0						27.4	29.0
AC 4% PIK Note [Member]
Subsequent Event [Line Items]
Debt instrument, interest rate														4.00%										4.00%					4.00%
Repayment of debt																								$ 0	$ 50,000
Loan from AC [Member]
Subsequent Event [Line Items]
Debt instrument, interest rate																												1.60%
AC 1.6% Note [Member]
Subsequent Event [Line Items]
Debt instrument, interest rate														1.60%										1.60%			1.60%
Repayment of debt																								$ 0	$ 15,000
Class A [Member]
Subsequent Event [Line Items]
Common stock, shares outstanding (in shares) \| shares														8,356,290				9,957,301						8,356,290	9,957,301
Restricted Stock Awards [Member]
Subsequent Event [Line Items]
Number of shares with accelerated vesting (in shares) \| shares												19,400
RSAs granted (in shares) \| shares					264,900			5,000	162,450		270,500
Restricted Stock Awards [Member] \| Vesting in Three Years from Date of Grant [Member]
Subsequent Event [Line Items]
Award vesting percentage																								30.00%
Award vesting period																								3 years
Restricted Stock Awards [Member] \| Vesting in Five Years from Date of Grant [Member]
Subsequent Event [Line Items]
Award vesting percentage																								70.00%
Award vesting period																								5 years
Restricted Stock Awards [Member] \| Mr. Gabelli [Member] \| Award Granted for 2016 [Member] \| Class A [Member]
Subsequent Event [Line Items]
Deferred compensation agreement, value of shares issued																										$ 76,000
Deferred compensation agreement, share price (in dollars per share) \| $ / shares																								$ 32.8187		$ 32.8187
Subsequent Event [Member]
Subsequent Event [Line Items]
Stock repurchased (in shares) \| shares													55,093
Average price per share of repurchased shares (in dollars per share) \| $ / shares													$ 17.16
Shares available to be repurchased under the plan (in shares) \| shares													1,188,042
Subsequent Event [Member] \| Quarterly Dividend Declared in Q1 2020 [Member]
Subsequent Event [Line Items]
Dividends declared date		Feb. 04, 2020
Dividends declared (in dollars per share) \| $ / shares		$ 0.02
Dividends payable date		Mar. 31, 2020
Dividends record date		Mar. 17, 2020
Subsequent Event [Member] \| Restricted Stock Awards [Member]
Subsequent Event [Line Items]
RSAs granted (in shares) \| shares	392,700
Subsequent Event [Member] \| Restricted Stock Awards [Member] \| Vesting in Three Years from Date of Grant [Member]
Subsequent Event [Line Items]
Award vesting percentage	30.00%
Subsequent Event [Member] \| Restricted Stock Awards [Member] \| Vesting in Five Years from Date of Grant [Member]
Subsequent Event [Line Items]
Award vesting percentage	70.00%
Subsequent Event [Member] \| Restricted Stock Awards [Member] \| Mr. Gabelli [Member] \| Award Granted for 2016 [Member] \| Class A [Member]
Subsequent Event [Line Items]
Payments for compensation agreement			$ 43,700
Reduction of RSU expense due to cap and DCCA being indexed to GBL stock price			$ 32,300
Deferred compensation agreement, share price (in dollars per share) \| $ / shares			$ 18.8812