Information by Business Segment (Details 4) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Reconciliation of segment assets to total assets
Assets	$ 3,596,238	$ 3,595,205	$ 3,757,131
Segment assets
Reconciliation of segment assets to total assets
Assets	3,025,966		2,970,117
Non-operating property assets
Reconciliation of segment assets to total assets
Assets	425,951		429,690
Other assets
Reconciliation of segment assets to total assets
Assets	$ 144,321		$ 357,324