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Senior Notes - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Debt Instrument [Line Items]      
Average interest rate 5.30% 5.80%  
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. The Company was in compliance with all of the financial and other covenants.  
(Loss)gain on debt retirement, net 0 (13,495,000) (89,000)
Senior Unsecured Notes due May 2023 [Member]
      
Debt Instrument [Line Items]      
Senior note principal amount 300,000,000    
Interest rate of senior notes 3.375%    
Discount rate of Senior notes 99.397%    
Debt Instrument, Maturity Date May 15, 2023    
Senior Unsecured Notes due January 2021 [Member]
      
Debt Instrument [Line Items]      
Senior note principal amount 300,000,000    
Interest rate of senior notes 3.50%    
Discount rate of Senior notes 99.327%    
Debt Instrument, Maturity Date Jan. 15, 2021    
Senior Convertible Notes due 2040 [Member]
      
Debt Instrument [Line Items]      
Debt Instrument, Maturity Date Nov. 15, 2040    
Conversion price $ 15.33 $ 15.76  
Trading days, minimum 20 days    
Trading days, maximum 30 days    
Conversion price per share greater than applicable trading day 125.00%    
Remaining amortization periods for debt discount 22 months    
Senior Unsecured Notes [Member]
      
Debt Instrument [Line Items]      
Aggregate Extinguishment of Debt, Amount   60,000,000 36,100,000
(Loss)gain on debt retirement, net   (13,495,000) (89,000)
Senior Convertible Notes [Member]
      
Debt Instrument [Line Items]      
Senior note principal amount 350,000,000 350,000,000  
Senior convertible notes repurchased     $ 19,400,000