Income Taxes (Net Income Reconciliation) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Income Tax Disclosure [Abstract]
GAAP net income	$ 29,273	$ 19,993	$ 1,638	$ 10,346	$ 6,911	$ 2,093	$ 23,669	$ (2,081)	$ 61,250		$ 30,592		$ 5,328
GAAP net income of taxable REIT subsidiaries and noncontrolling redeemable interest, net									15		1,224		3,002
GAAP net income from REIT operations									61,265	[1]	31,816	[1]	8,330	[1]
Book depreciation and amortization									61,834		57,672		55,899
Tax depreciation and amortization									(41,661)		(39,835)		(35,086)
Book/tax differences on gains from capital transactions									(50,472)		(19,128)		(4,677)
Other book/tax differences, net									(4,616)		1,088		51
Taxable income before adjustments									26,350		31,613		24,517
Capital gain									(2,356)		(7,762)		(9,920)
Taxable income subject to dividend requirements									$ 23,994		$ 23,851		$ 14,597

[1] All adjustments to GAAP net income from REIT operations are net of amounts attributable to taxable REIT subsidiaries and noncontrolling interests.