Partners' Capital - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended			2 Months Ended	3 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 06, 2014	Dec. 31, 2012	May 27, 2004	Apr. 11, 2013	Dec. 21, 2012	Jan. 13, 2006	Aug. 21, 2006	Mar. 13, 2014	Mar. 01, 2012
Class of Stock [Line Items]
Operating Partnership, aggregate cash payment									$ 89,943
Operating Partnership, shares									4,374,637
Liquidation Preference	0				75,000				0	75,000
Cost associated with the redemptions	0	0	0	1,462	0	2,121	3,546	0	1,462	5,667	1,804
Preferred stock shares authorized	10,000,000								10,000,000
Converted Units									222,676	105,028	535,026
Reclassifications of Partner's Capital									2,155	996	4,763
Net proceeds from the issuance of common stock									0	174,081	134,905
Series F Preferred Units
Class of Stock [Line Items]
Liquidation Preference					50,000					50,000
Depositary shares issued														50,000
Depositary Share percentage per share														1.00%
Dividend rate per share									6.236%			6.275%
Par value per share														$ 0.01
Initial offering price per depositary share														$ 1,000.00
Aggregate redemption price														50,000
Dividend per Depositary share				$ 11.3299
Total dividends, Depositary Shares				566
Cost associated with the redemptions									949
Minimum fixed coupon rate of preferred units	2.375%								2.375%
Number of years of U.S. treasury rate one												30 years
Number of years of U.S. treasury rate two												10 years
LIBOR rate												3-month LIBOR
Series G Preferred Units
Class of Stock [Line Items]
Liquidation Preference					25,000					25,000
Depositary shares issued														25,000
Depositary Share percentage per share														1.00%
Dividend rate per share									7.236%
Par value per share														$ 0.01
Initial offering price per depositary share														$ 1,000.00
Redemption price per share														$ 1,000.00
Aggregate redemption price														25,000
Dividend per Depositary share				$ 36.18
Total dividends, Depositary Shares				905
Cost associated with the redemptions									513
Series J Preferred Units
Class of Stock [Line Items]
Depositary shares issued																	6,000,000
Depositary Share percentage per share																	0.01%
Dividend rate per share									7.25%
Par value per share																	$ 0.01
Initial offering price per depositary share																	$ 25.00
Redemption price per share																	$ 25.00
Aggregate redemption price																	150,000
Dividend per Depositary share							$ 0.055382						$ 0.407812
Total dividends, Depositary Shares							221						816
Cost associated with the redemptions										3,546	1,804
Depositary Shares redeemed															4,000,000	2,000,000
Series K Preferred Units
Class of Stock [Line Items]
Depositary shares issued																		2,000,000
Depositary Share percentage per share																		0.01%
Dividend rate per share									7.25%
Par value per share																		$ 0.01
Initial offering price per depositary share																		$ 25.00
Redemption price per share																		$ 25.00
Aggregate redemption price																		50,000
Dividend per Depositary share						$ 0.090625
Total dividends, Depositary Shares						181
Cost associated with the redemptions										2,121
Common Stock
Class of Stock [Line Items]
Shares of common stock issued										8,400,000	9,400,000
Net proceeds from the issuance of common stock										132,050	116,715
ATM
Class of Stock [Line Items]
Shares of common stock issued										2,315,704	1,532,598
Net proceeds from the issuance of common stock										41,735	18,063
Shares of company stock issuable under the at-the-market offering																			13,300,000	12,500,000
Aggregate gross sale proceeds																			$ 200,000	$ 125,000