Dividends and Equity Issuance Program - Additional Information (Detail) - USD ($)	3 Months Ended		42 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Jun. 30, 2016
Dividends and Equity Issuance [Line Items]
Dividends declared and paid	$ 8,907,000	$ 8,596,000
Declared and paid dividends, per share	$ 0.655	$ 0.645
Securities, aggregate sales price				$ 23,300,000
At-The-Market (ATM) Equity Issuance Program
Dividends and Equity Issuance [Line Items]
Share issued	0		829,916
Average sale price per share			$ 48.77
Net cash proceeds from stock issued			$ 38,800,000
Payment of stock issuance cost			1,700,000
Payments for stock issuance			1,000,000
At-The-Market (ATM) Equity Issuance Program | Other Expense
Dividends and Equity Issuance [Line Items]
Payments for stock issuance			$ 680,000