Dividends and Equity Issuance Program - Additional Information (Detail) - USD ($)	3 Months Ended			9 Months Ended		36 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016
Dividends and Equity Issuance [Line Items]
Dividends declared and paid	$ 8,800,000		$ 8,500,000	$ 26,231,000	$ 25,494,000
Declared and paid dividends, per share	$ 0.65		$ 0.64	$ 1.945	$ 1.915
Securities, aggregate sales price		$ 23,300,000
At-The-Market (ATM) Equity Issuance Program
Dividends and Equity Issuance [Line Items]
Share issued		249,016		249,016		829,916
Average sale price per share		$ 55.30		$ 55.30		$ 48.77
Net cash proceeds from stock issued				$ 13,200,000		$ 38,800,000
Payment of stock issuance cost				558,000		1,700,000
Payments for stock issuance				344,000		1,000,000
At-The-Market (ATM) Equity Issuance Program | Other Expense
Dividends and Equity Issuance [Line Items]
Payments for stock issuance				$ 214,000		$ 680,000