Supplemental Disclosure to Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Supplemental Cash Flow Information [Abstract]
Write-off of fully depreciated/amortized property and deferred costs	$ 12,685	$ 14,436
Write-off of depreciated/amortized property and deferred costs due to sale	32,323	10,295
Unrealized loss on cash flow hedge	(1,360)	(947)
Changes in accrued development capital expenditures	$ 394	$ (8,343)