Notes Receivable - Additional Information (Detail) - USD ($)		3 Months Ended	12 Months Ended
	Apr. 30, 2005	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total other-than-temporary impairment loss			$ 41,961,000
Net impairment loss recognized in earnings on held-to-maturity securities		$ 35,400,000	35,418,000
Less portion recognized in other comprehensive income			6,543,000
Interest income			11,818,000	$ 11,500,000	$ 12,384,000
National Bonds [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income			11,600,000	11,400,000	12,300,000
Payment received relating to notes receivables			11,100,000	$ 11,100,000	$ 9,400,000
Bonds A Series [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Bond issued, face value		95,000,000	$ 95,000,000
Interest Rates on Bonds			8.00%
Maturity date of notes receivable			Jul. 01, 2034
Aggregate carrying values		80,300,000	$ 80,300,000
Effective interest rates on bonds	8.04%
Present value of future debt service payments discounted	$ 93,800,000
Bonds B Series [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Bond issued, face value		50,000,000	$ 50,000,000
Interest Rates on Bonds			10.00%
Maturity date of notes receivable			Sep. 01, 2037
Aggregate carrying values		$ 31,200,000	$ 31,200,000
Effective interest rates on bonds	11.42%		14.00%
Present value of future debt service payments discounted	$ 38,300,000
Net impairment loss recognized in earnings on held-to-maturity securities			$ 35,400,000
Less portion recognized in other comprehensive income			$ 6,500,000