Shareholders' Equity - Additional Information (Detail)		3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 03, 2015 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) shares	Jun. 30, 2014 shares	Jun. 30, 2015 USD ($) Facility $ / shares shares	Jun. 30, 2014 USD ($) $ / shares shares	Dec. 31, 2014 Facility	May. 12, 2014 USD ($)
Stockholders Equity [Line Items]
Common stock shares issued under equity offering program | shares	1,380,000
Common stock, price per share, public offering | $ / shares	$ 90.40
Proceeds from issuance of common stock	$ 119,500,000
Common stock issued upon exercise of stock options | shares		5,500	7,500	15,000	9,000
Common stock value authorized under equity offering program							$ 225,000,000
Number of facilities acquired | Facility				15		33
Shares issued under dividend reinvestment plan | shares				73,186
Equity Program [Member]
Stockholders Equity [Line Items]
Common stock shares issued under equity offering program | shares				199,700	250,000
Proceeds from issuance of common stock				$ 18,000,000	$ 45,400,000
Common stock, value reserved for future issuance		$ 133,000,000		$ 133,000,000
Weighted average issue price of common stock | $ / shares				$ 91.53	$ 75.45
Number of facilities acquired | Facility				15
Equity Program [Member] | Jefferies Llc [Member]
Stockholders Equity [Line Items]
Offering related costs and commissions				$ 200,000
Equity Program [Member] | SunTrust [Member]
Stockholders Equity [Line Items]
Offering related costs and commissions					$ 300,000
Equity Program [Member] | Piper Jaffray & Co [Member]
Stockholders Equity [Line Items]
Offering related costs and commissions					$ 300,000
Previous Equity Program [Member]
Stockholders Equity [Line Items]
Common stock shares issued under equity offering program | shares					359,102