Financing Agreement - Additional Information (Detail) (USD $)	1 Months Ended
	Dec. 31, 2010	Dec. 31, 2013	Jan. 09, 2015 sqft	Mar. 31, 2015	Dec. 31, 2014	Aug. 31, 2011
Debt Instrument [Line Items]
Area of Real Estate Property			600,000
Senior Notes | Senior Unsecured Notes 3.81% Due May 2018
Debt Instrument [Line Items]
Proceeds from Issuance of Debt	$ 25,000,000
Debt Instruments Maturity Date	5/31/2018
Mortgage note payable				25,000,000	25,000,000
Senior Notes | Senior Unsecured Notes 4.60% Due May 2021
Debt Instrument [Line Items]
Proceeds from Issuance of Debt	75,000,000
Debt Instruments Maturity Date	5/31/2021
Mortgage note payable				75,000,000	75,000,000
Senior Notes | Senior Unsecured Notes 3.13% Due December 2020
Debt Instrument [Line Items]
Proceeds from Issuance of Debt		100,000,000
Debt Instruments Maturity Date		12/31/2020
Mortgage note payable				100,000,000	100,000,000
Notes payable
Debt Instrument [Line Items]
Mortgage note payable				14,500,000	0
Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 500,000,000		$ 350,000,000