Distributions - Summary of Distributions Paid Including Activity in Plan as well as Limited Partnership Units Issued from Reinvestment of Unit Distributions (Detail) - USD ($) $ / shares in Units, $ in Thousands												1 Months Ended	12 Months Ended
	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2017	Oct. 31, 2016	Jul. 31, 2016	Apr. 30, 2016	Jan. 31, 2016	Oct. 31, 2015	Jul. 31, 2015	Apr. 30, 2015	Jan. 31, 2015	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends [Line Items]
Preferred Stockholders													$ 12,375	$ 12,375	$ 12,375
Common Stockholders													44,576	39,472	35,645
Limited Partnership Unitholders													$ 15,268	$ 13,533	$ 12,228
Common Stock Shares Issued (in shares)													266,011	186,797	201,212
Limited Partnership Units Issued (in shares)													111,351	124,758	107,037
October 31
Dividends [Line Items]
Preferred Stockholders	$ 3,094			$ 3,094				$ 3,094
Common Stockholders	11,221			10,168				9,106
Limited Partnership Unitholders	$ 3,838			$ 3,478				$ 3,129
Common Stock Shares Issued (in shares)	82,991			44,176				47,313
Discounted Share Price (in usd per share)	$ 59.33			$ 57.18				$ 55.73
Limited Partnership Units Issued (in shares)	15,596			30,891				28,936
Average Unit Price (in usd per share)	$ 60.08			$ 57.18				$ 55.73
July 31
Dividends [Line Items]
Preferred Stockholders		$ 3,094			$ 3,094				$ 3,094
Common Stockholders		11,160			10,133				9,081
Limited Partnership Unitholders		$ 3,830			$ 3,465				$ 3,115
Common Stock Shares Issued (in shares)		85,731			39,487				56,003
Discounted Share Price (in usd per share)		$ 57.40			$ 65.64				$ 50.30
Limited Partnership Units Issued (in shares)		16,021			26,897				32,041
Average Unit Price (in usd per share)		$ 58.13			$ 65.64				$ 50.30
April 30
Dividends [Line Items]
Preferred Stockholders			$ 3,094			$ 3,094				$ 3,094
Common Stockholders			11,119			10,029				9,055
Limited Partnership Unitholders			$ 3,810			$ 3,449				$ 3,104
Common Stock Shares Issued (in shares)			51,003			48,854				54,921
Discounted Share Price (in usd per share)			$ 59.64			$ 51.59				$ 50.21
Limited Partnership Units Issued (in shares)			40,623			34,201				25,264
Average Unit Price (in usd per share)			$ 59.96			$ 51.59				$ 50.21
January 31
Dividends [Line Items]
Preferred Stockholders							$ 3,093				$ 3,093	$ 3,093
Common Stockholders							9,142				8,403	11,076
Limited Partnership Unitholders							$ 3,141				$ 2,880	$ 3,790
Common Stock Shares Issued (in shares)							54,280				42,975	46,286
Discounted Share Price (in usd per share)							$ 49.24				$ 56.74	$ 61.85
Limited Partnership Units Issued (in shares)							32,769				20,796	39,111
Average Unit Price (in usd per share)							$ 49.24				$ 56.74	$ 62.15