Reclassification - Schedule of Consolidated Statements of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues
Rental income	$ 83,487	$ 74,394	$ 162,373	$ 144,347	$ 302,044	$ 253,384	$ 217,906
Other operating income	7,239	6,050	13,761	11,555	24,036	20,123	16,176
Total operating revenues	90,726	80,444	176,134	155,902	326,080	273,507	234,082
Expenses
Property operations and maintenance	19,486	17,993	40,046	36,478	75,333	66,119	59,011
Real estate taxes	9,145	8,055	18,064	16,121	32,097	26,496	22,076
General and administrative	9,686	8,864	19,092	17,400	35,222	30,136	28,465
Acquisition costs	788	1,938	1,369	4,716	7,359	3,129	4,328
Operating leases of storage facilities		1,997	683	3,994	7,987	1,331
Depreciation and amortization	14,584	12,481	28,766	24,423	51,749	45,233	40,542
Total operating expenses	53,689	51,328	108,020	103,132	209,747	172,444	154,422
Income from operations	37,037	29,116	68,114	52,770	116,333	101,063	79,660
Other income (expenses)
Interest expense	(9,216)	(8,872)	(18,377)	(16,216)	(34,578)	(32,000)	(33,166)
Interest income	2	24	4	31	40	40	4
Gain on sale of storage facilities			(7)		5,176
Gain on sale of real estate						421	687
Equity in income of joint ventures	853	433	1,499	892	2,086	1,948	936
Income from continuing operations					89,057	71,472	48,121
Income from discontinued operations (including a gain on disposal of $2,431 in 2013 and $4,498 in 2012)						3,123	7,520
Net income	28,676	20,701	51,233	37,477	89,057	74,595	55,641
Net income attributable to noncontrolling interest	(144)	(125)	(250)	(228)	(526)	(469)	(513)
Net income attributable to common shareholders	$ 28,532	$ 20,576	$ 50,983	$ 37,249	$ 88,531	$ 74,126	$ 55,128
Earnings per common share attributable to common shareholders-basic
Continuing operations					$ 2.68	$ 2.27	$ 1.62
Discontinued operations						0.10	0.26
Earnings per share-basic	$ 0.81	$ 0.63	$ 1.46	$ 1.14	2.68	2.37	1.88
Earnings per common share attributable to common shareholders-diluted
Continuing operations					2.67	2.26	1.61
Discontinued operations						0.10	0.26
Earnings per share-diluted	$ 0.80	$ 0.62	$ 1.45	$ 1.14	$ 2.67	$ 2.36	$ 1.87