Equity - Additional Information (Detail) (USD $)	1 Months Ended					3 Months Ended		6 Months Ended				1 Months Ended						1 Months Ended				6 Months Ended	1 Months Ended				1 Months Ended				1 Months Ended													1 Months Ended		1 Months Ended
	Jun. 20, 2013	Jun. 03, 2013	Apr. 30, 2013	Mar. 15, 2013	Mar. 01, 2013	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Nov. 21, 2012 Capital on Demand Sales Agreement Maximum	Jan. 29, 2013 Restricted Stock	Jan. 29, 2013 Restricted Stock Non-management trustees Person	Jan. 29, 2013 Restricted Stock Executive Officers	Jan. 29, 2013 Restricted Stock Employees	Jan. 29, 2013 Stock Appreciation Rights (SARs)	Jun. 30, 2013 Dividend Reinvestment And Share Purchase Plan	May 14, 2013 7.75% Series A cumulative redeemable preferred shares	Jan. 29, 2013 7.75% Series A cumulative redeemable preferred shares	Jun. 30, 2013 7.75% Series A cumulative redeemable preferred shares	Dec. 31, 2012 7.75% Series A cumulative redeemable preferred shares	Jun. 30, 2013 7.75% Series A cumulative redeemable preferred shares At Market Issuance Sales Agreement	May 14, 2013 8.375% Series B cumulative redeemable preferred shares	Jan. 29, 2013 8.375% Series B cumulative redeemable preferred shares	Jun. 30, 2013 8.375% Series B cumulative redeemable preferred shares	Dec. 31, 2012 8.375% Series B cumulative redeemable preferred shares	May 14, 2013 8.875% Series C cumulative redeemable preferred shares	Jan. 29, 2013 8.875% Series C cumulative redeemable preferred shares	Jun. 30, 2013 8.875% Series C cumulative redeemable preferred shares	Dec. 31, 2012 8.875% Series C cumulative redeemable preferred shares	May 14, 2013 Series D Preferred Stock	Jan. 29, 2013 Series D Preferred Stock	May 14, 2013 Preferred Stock	Jan. 29, 2013 Preferred Stock	Jun. 20, 2013 Common Stock	Mar. 15, 2013 Common Stock	Jul. 31, 2013 Subsequent Event	Aug. 05, 2013 Subsequent Event Capital on Demand Sales Agreement	Jul. 30, 2013 Subsequent Event Restricted Stock Non-management trustees Person	Jul. 30, 2013 Subsequent Event 7.75% Series A cumulative redeemable preferred shares	Aug. 05, 2013 Subsequent Event 7.75% Series A cumulative redeemable preferred shares	Jul. 30, 2013 Subsequent Event 8.375% Series B cumulative redeemable preferred shares	Aug. 05, 2013 Subsequent Event 8.375% Series B cumulative redeemable preferred shares	Jul. 30, 2013 Subsequent Event 8.875% Series C cumulative redeemable preferred shares	Aug. 05, 2013 Subsequent Event 8.875% Series C cumulative redeemable preferred shares	Jul. 30, 2013 Subsequent Event Series D Preferred Stock	Jul. 30, 2013 Subsequent Event Preferred Stock
Class of Stock [Line Items]
Preferred Shares, dividend declared																		$ 0.484375	$ 0.484375				$ 0.5234375	$ 0.5234375			$ 0.5546875	$ 0.5546875			$ 0.4687500	$ 0.4687500								$ 0.484375		$ 0.5234375		$ 0.5546875		$ 0.4687500
Dividend payment date																																	Jul. 01, 2013	Apr. 01, 2013	Jul. 31, 2013	Apr. 30, 2013											Sep. 30, 2013
Dividend record date																																	Jun. 03, 2013	Mar. 01, 2013	Jul. 12, 2013	Apr. 03, 2013											Sep. 03, 2013
Preferred Shares, dividend paid		$ 5,142			$ 4,840
Preferred shares authorized to be issued under agreement						1,400,000		1,400,000												2,000,000					2,000,000				2,000,000
Cumulative redeemable preferred shares, shares issued																				3,749,288	3,124,288	625,000			2,288,465	2,288,465			1,640,100	1,640,100											320,000
Weighted-average price per share																						$ 24.00																$ 7.59			$ 23.75
Net proceeds received from issuance of Preferred Shares																						14,550,000																			7,372,000
Aggregate of Series A,B,C Preferred Shares available for issuance																																									690,712		1,969,835		1,959,900
Common stock, dividend declared	$ 0.13			$ 0.12		$ 0.13	$ 0.08	$ 0.25	$ 0.16
Common stock, dividend paid			8,334																																		9,032
Number of phantom unit awards redeemed for common shares								54,728
Number of shares awarded, share													43,536	369,500	369,500	1,127,500																							6,578
Number of shares awarded, value													300,000	2,577,000	2,577,000	1,332,000																							50,000
Closing stock price													$ 6.89	$ 6.89	$ 6.89																								$ 7.60
Number of individuals to whom shares are awarded													6																										1
Remaining awards vesting period													9 months																										3 months
Vesting period												3 years				3 years
Expiration date																Jan. 29, 2018
Registered and reserved common shares																	10,500,000
Common shares pursuant to the DRSPP								2,343
Weighted-average price per shares																	$ 7.81
Net proceeds received								18,000
Common shares remaining available under the DRSPP						7,781,137		7,781,137
Common shares to be issued and sold											10,000,000
Common shares issued						69,960,454		69,960,454		58,913,142																												287,277
Net proceeds received from issuance of common shares								76,749,000	41,535,000																													2,126,000
Common shares remaining available for issuance																																						9,712,723
Common shares issued for exercise of underwriters option to purchase common shares						1,350,000		1,350,000
Net proceeds received during period from common share issuance in underwritten public offering			$ 70,196,000					$ 7,358,000
Common shares in an underwritten public offering			9,200,000
Public offering price			$ 7.87