Partners' Capital - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended				3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 ATM [Member]	Mar. 01, 2012 ATM [Member]	Jun. 30, 2013 Restricted Stock [Member]	Jun. 30, 2012 Restricted Stock [Member]	Jun. 30, 2013 Restricted Stock [Member]	Jun. 30, 2012 Restricted Stock [Member]	Jun. 30, 2013 Series F Preferred Units [Member]
Class of Stock [Line Items]
Depositary shares redeemed			4,000,000
Portion of preferred stock	0.0001		0.0001
Dividend rate per share			7.25%
Par value of each depositary share	$ 0.01		$ 0.01
Redemption price per share	$ 25.00		$ 25.00
Preferred dividend per depositary share			$ 0.055382
Total preferred dividend, depositary shares			$ 221,000
Cost associated with share redemption	3,546,000	0	3,546,000	0
Underwritten public offering shares			8,400,000
Net proceeds from underwriting			132,050,000
Shares of common stock issuable under at-the-market offering						12,500,000
Common stock issued under at-the-market offering					2,315,704
Aggregate gross sale proceeds						125,000
Proceeds from Issuance of common stock					41,770,000
Conversion of units to common stock			85,028
Reclassification between limited and general units due to conversion			804,000
Restricted stock/unit awards to employees									284,461
Fair value of restricted stock/unit awards									4,719,000
Vesting period of restricted stock/unit awards to employees									3 years
Compensation expense related to restricted shares issued during the quarter			1,008,000
Amortization related to restricted stock and unit awards							841,000	1,299,000	2,667,000	2,398,000
Amount of share based compensation expense capitalized							4,000		13,000
Unrecognized compensation expense related to restricted stock/unit awards							$ 5,452,000		$ 5,452,000
Weighted average period during which unrecognized compensation expense is expected to be recognized									1 year 0 months 20 days
Minimum fixed coupon rate of preferred stock											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
New coupon rate of preferred stock											5.515%