Notes Receivable - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income	$ 2,766	$ 2,969	$ 5,519	$ 5,917
National Bonds [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income	$ 2,700	$ 2,900	5,300	5,900
Payment received relating to notes receivables			$ 3,100	$ 3,200
Bonds A Series [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maturity date of notes receivable					Jul. 01, 2034
Bonds B Series [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maturity date of notes receivable					Sep. 01, 2037