Income Taxes: (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Common stock dividend distribution
Ordinary income (in dollars per share)	$ 0.74	$ 0.85	$ 0.57
Capital gains (in dollars per share)	$ 1.13	$ 0.01	$ 0.04
Unrecaptured Section 1250 gain (in dollars per share)	$ 0.36	$ 0.04
Return of capital (in dollars per share)		$ 1.15	$ 1.49
Dividends paid (in dollars per share)	$ 2.23	$ 2.05	$ 2.10
Ordinary income (as a percent)	33.20%	41.50%	27.10%
Capital gains (as a percent)	50.70%	0.50%	1.90%
Unrecaptured Section 1250 gain (as a percent)	16.10%	2.00%
Return of capital (as a percent)		56.00%	71.00%
Dividends paid (as a percent)	100.00%	100.00%	100.00%
Income tax benefit
Current			$ (11)
Deferred	4,159	6,110	9,213
Income tax benefit	4,159	6,110	9,202
Reconciliation of income tax benefit (provision) of the TRSs
Book loss for TRSs	16,154	19,558	19,896
Tax at statutory rate on earnings from continuing operations before income taxes	5,493	6,650	6,765
Other	(1,334)	(540)	2,437
Income tax benefit	4,159	6,110	9,202
Components of net deferred tax assets
Net operating loss carryforwards	33,781	29,045
Property, primarily differences in depreciation and amortization, the tax basis of land assets and treatment of certain other assets costs	(1,973)	(4,442)
Other	1,606	2,226
Net deferred tax assets	33,414	26,829
Reconciliation of unrecognized tax benefits
Unrecognized tax benefits at beginning of year			2,420
Gross decreases for tax positions of current year			$ (2,420)