Earnings Per Share and Dividends - Additional Information (Details) - USD ($)		3 Months Ended				6 Months Ended
	Jul. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Dividends Payable [Line Items]
Cash dividends declared per share of Class A common stock						$ 0.42	$ 0.40
Dividends declared and paid		$ 3,739,000	$ 3,740,000	$ 3,549,000	$ 3,547,000	$ 7,479,000	$ 7,096,000
Common Class A
Dividends Payable [Line Items]
Cash dividends declared per share of Class A common stock		$ 0.21	$ 0.21	$ 0.20	$ 0.20	$ 0.42	$ 0.40
Dividends declared and paid		$ 0	$ 0	$ 0	$ 0
Quarterly dividend | Common Class A
Dividends Payable [Line Items]
Cash dividends declared per share of Class A common stock	$ 0.21
Non-controlling interest
Dividends Payable [Line Items]
Dividends declared and paid		0	0	0	0
Distributions declared to non-controlling unitholders		$ 2,638,000	$ 2,638,000	$ 2,512,000	$ 2,512,000	$ 5,276,000	$ 5,024,000