Equity Based Compensation Plans, Long term incentive plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	1,614	1,540	14,415
Series Z Incentive Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Basis for estimating fair value of a unit	The estimated fair value of a Z Unit is determined on the grant date and considers the company's current stock price, the dividends that are not paid on unvested units and a marketability discount for the 8 to 15 years of illiquidity.
Basis for calculating compensation expense	Compensation expense is calculated by multiplying estimated vesting increases for the period by the estimated fair value as of the grant date less its $1.00 per unit purchase price.
Maximum number of shares (units) authorized for issuance (in shares)	600,000
Conversion ratchet increase for shares issued prior to 2010	Effective January 1 of each year for each participating executive who remains employed by the Company if the Company has met a specified "funds from operations" per share target, or such other target as the Compensation Committee deems appropriate, for the prior year, up to a maximum conversion ratchet of 100%.
Granted (in shares)	0	46,500	108,000
Number of senior executives receiving partnership units in exchange for capital committment		6
Capital commitment per series of each incentive Unit received (in dollars per share)		1.00
Number of senior executives receiving partnership units		14
Number of senior executives receiving partnership units in exchange for cash		8
Incentive units conversion ratio		one-for-one
Incentive units conversion threshold		Convertible into common stock of the Company upon the earlier to occur of 100 percent vesting of the units or the year 2026.
Conversion ratchet increase for shares issued in 2010		The conversion ratchet (accounted for as vesting) of the 2011 Z-1 Units into common units, increased to 10 percent effective January 1, 2012 because the Company achieved the FFO minimum target of $5.65 per diluted share in 2011. Each year thereafter, vesting of the 2011 Z-1 Units will be consistent with the Company's annual FFO growth, but is not to be less than zero or greater than 14 percent.
Percentage unit holders entitlement to dividends distributed to common stockholders (in hundredths)		10.00%
Funds from operations (FFO) target per diluted share (in dollars per share)		5.65
2010 Series Z-1 Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Funds from operations (FFO) target per diluted share (in dollars per share)			4.75