Notes Receivable - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income	$ 2,753	$ 2,948
National Bonds [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income	2,700	2,900
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