Income Taxes: (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common stock dividend distribution
Ordinary income (in dollars per share)	$ 1.92	$ 1.02	$ 0.74
Capital gains (in dollars per share)	$ 0.16	$ 1.24	$ 1.13
Unrecaptured Section 1250 gain (in dollars per share)	$ 0.05	$ 0.10	$ 0.36
Return of capital (in dollars per share)	$ 0.38	$ 0.00	$ 0.00
Dividends paid (in dollars per share)	$ 2.51	$ 2.36	$ 2.23
Ordinary income (as a percent)	76.50%	43.30%	33.20%
Capital gains (as a percent)	6.40%	52.50%	50.70%
Unrecaptured Section 1250 gain (as a percent)	2.00%	4.20%	16.10%
Return of capital (as a percent)	15.10%	0.00%	0.00%
Dividends paid (as a percent)	100.00%	100.00%	100.00%
Income tax benefit
Current	$ 0	$ (142)	$ 0
Deferred	4,269	1,834	4,159
Income tax benefit	4,269	1,692	4,159
Reconciliation of income tax benefit (provision) of the TRSs
Book loss for TRSs	10,785	11,709	16,154
Tax at statutory rate on earnings from continuing operations before income taxes	3,667	3,981	5,493
Other	602	(2,289)	(1,334)
Income tax benefit	4,269	1,692	4,159
Components of net deferred tax assets
Net operating loss carryforwards	24,698	26,394
Property, primarily differences in depreciation and amortization, the tax basis of land assets and treatment of certain other costs	8,201	3,673
Other	2,726	1,289
Deferred tax assets	$ 35,625	$ 31,356