Information by Business Segment (Details 3) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Reconciliation of segment assets to total assets      
Assets $ 3,777,155 $ 3,670,257 $ 3,605,897
Segment assets      
Reconciliation of segment assets to total assets      
Assets 3,110,380   2,932,749
Non-operating property assets      
Reconciliation of segment assets to total assets      
Assets 496,930   503,030
Other assets      
Reconciliation of segment assets to total assets      
Assets $ 169,845   $ 170,118