Dividends and Equity Issuance Program - Additional Information (Detail) - USD ($)	3 Months Ended		30 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016
Dividends and Equity Issuance [Line Items]
Dividends declared and paid	$ 8,596,000	$ 8,446,000
Declared and paid dividends, per share	$ 0.645	$ 0.635
Securities, aggregate sales price	$ 23,300,000		$ 23,300,000
At-The-Market (ATM) Equity Issuance Program
Dividends and Equity Issuance [Line Items]
Share issued	0		580,900
Average sale price per share			$ 45.97
Net cash proceeds from stock issued			$ 25,600,000
Payment of stock issuance cost		$ 1,100,000	1,100,000
Payments for stock issuance			667,000
At-The-Market (ATM) Equity Issuance Program | Other Expense
Dividends and Equity Issuance [Line Items]
Payments for stock issuance			$ 391,000