Senior Notes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Average interest rate	5.30%	5.30%
Notes issued which may not be redeemed prior to maturity	$ 82,200,000
Covenant terms	The fixed-rate senior notes and senior convertible notes were issued pursuant to indentures that contain certain covenants, including limitation on incurrence of debt, maintenance of unencumbered real estate assets and debt service coverage. The covenants also require that the cumulative dividends declared or paid from December 31, 1993, through the end of the current period cannot exceed Funds From Operations (as defined in the agreement) plus an additional $20.0 million for the same period unless required to maintain REIT status.
Covenant compliance	The Company was in compliance with all of the financial and other covenants.
(Loss) gain on debt retirement, net	0	0	(13,495,000)
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Aggregate Extinguishment of Debt, Amount			60,000,000
(Loss) gain on debt retirement, net			(13,495,000)
Total fees paid for the issuance of senior notes		$ 1,300,000	$ 1,300,000
Senior Convertible Notes due 2040 [Member]
Debt Instrument [Line Items]
Conversion price	$ 14.85	$ 15.33
Trading days, minimum	20 days
Trading days, maximum	30 days
Conversion price per share greater than applicable trading day	125.00%
Remaining periods for debt accretion	10 months