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Subsequent Events (Details) (USD $)
 3 Months Ended 12 Months Ended 1 Months Ended 1 Months Ended
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2009
Partner
Dec. 31, 2011
Restricted stock
Dec. 31, 2010
Restricted stock
Dec. 31, 2011
Bank Borrowings
Mar. 31, 2012
Dividend declared
Feb. 29, 2012
Dividend declared
Jan. 31, 2012
Dividend declared
Dec. 31, 2011
Dividend declared
Nov. 30, 2011
Dividend declared
Oct. 31, 2011
Dividend declared
Sep. 30, 2011
Dividend declared
Aug. 31, 2011
Dividend declared
Jul. 31, 2011
Dividend declared
Jun. 30, 2011
Dividend declared
May 31, 2011
Dividend declared
Apr. 30, 2011
Dividend declared
Mar. 31, 2011
Dividend declared
Feb. 28, 2011
Dividend declared
Jan. 31, 2011
Dividend declared
Jan. 31, 2012
Repayment of debt
Bank Borrowings
Jan. 31, 2012
Issuance of debt
Bank Borrowings
Jan. 31, 2012
Issuance of shares
Restricted stock
Exercise Price Dollar 31.77 [Member]
Jan. 31, 2012
Issuance of shares
Vesting on June 15, 2015
Restricted stock
Exercise Price Dollar 31.77 [Member]
Jan. 31, 2012
Issuance of shares
Vesting on January 10, 2016
Restricted stock
Exercise Price Dollar 31.77 [Member]
Jan. 31, 2012
Conversion of noncontrolling interest
Partner
Subsequent Events                                                                  
Cash dividend per common share (in dollars per share) $ 0.42 $ 0.42 $ 0.42 $ 0.42 $ 0.41 $ 0.39 $ 0.39 $ 0.39         $ 0.145 $ 0.145 $ 0.145 $ 0.14 $ 0.14 $ 0.14 $ 0.14 $ 0.14 $ 0.14 $ 0.14 $ 0.14 $ 0.14 $ 0.14 $ 0.14 $ 0.14            
Increase in dividend declared per share per month (as a percent)                             3.60%                                    
Amount borrowed under Unsecured Credit Agreement                       $ 167,600,000                                 $ 4,000,000        
Amount outstanding under Unsecured Credit Agreement 56,000,000       37,700,000             56,000,000                               60,000,000          
Amount available for borrowing under Unsecured Credit Agreement                       154,000,000                               150,000,000          
Number of shares granted (in shares)                   6,000 208,591                                     14,000      
Price per share                                                           $ 31.77 $ 31.77 $ 31.77  
Vesting period (in years)                   three-year                                       five-year      
Additional shares granted (in shares)                                                             30,000 12,200  
Number of Limited Partners Exercising Conversion Rights                 1                                               2
Number of limited partners exchanging all of their 67,294 partnership units                                                                 1
Number of partnership units converted by first limited partner (in shares)                                                                 67,294
Number of partnership units converted by second limited partner (in shares)                                                                 22,000
Redemption notification of limited partnership units (in shares)                 67,294                                               89,294
Cash paid, redemption of non-controlling interest                                                                 2,764,000
Margin added to the common stock price per share on redemption date to determine amount to be paid to limited partner (in dollars per share)                                                                 $ 0.05
Excess of redemption value over the book value                                                                 $ 1,246,000