Earnings Per Share and Dividends (Details 2) - USD ($)								3 Months Ended		9 Months Ended
	Nov. 04, 2015	Sep. 03, 2015	Jun. 04, 2015	Apr. 08, 2015	Sep. 03, 2014	Jun. 05, 2014	Apr. 18, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Class A common stock
Dividends Payable [Line Items]
Dividends to Class A common stockholders										$ 22,454,000
Cash dividends declared per share								$ 0.1250	$ 0.0625	$ 1.8750	$ 0.1875
Quarterly dividend | Class A common stock
Dividends Payable [Line Items]
Dividends to Class A common stockholders		$ 1,542,000	$ 1,529,000	$ 1,500,000	$ 724,000	$ 724,000	$ 725,000			$ 4,571,000	$ 2,173,000
Cash dividends declared per share	$ 0.125									$ 0.125	$ 0.0625
Distributions declared to non-controlling unitholders		$ 2,217,000	$ 2,217,000	$ 2,217,000	$ 1,108,000	$ 1,108,000	$ 1,108,000
Special dividend | Class A common stock
Dividends Payable [Line Items]
Dividends to Class A common stockholders										$ 17,883,000
Cash dividends declared per share										$ 1.50
Distributions declared to non-controlling unitholders										$ 26,602,000
Subsequent Event | Quarterly distribution
Dividends Payable [Line Items]
Distributions declared to non-controlling unitholders	$ 2,217,000