Dividends and Equity Issuance Program - Additional Information (Detail) - USD ($)		3 Months Ended	12 Months Ended			39 Months Ended
	Jan. 31, 2015	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Dividends and Equity Issuance [Line Items]
Declared and paid dividends, per share			$ 2.60	$ 2.56	$ 2.52
Ordinary income per share			2.00	2.17	1.478
Capital gain per share			$ 0.60	$ 0.39	$ 1.042
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	0	426,187	829,916
Average sale price per share		$ 55.30	$ 55.30		$ 47.51	$ 48.77
Net cash proceeds from stock issued	$ 1,100,000		$ 13,200,000		$ 19,500,000	$ 38,800,000
Payment of stock issuance cost			558,000		700,000	1,700,000
Payments for stock issuance			344,000		506,000	1,000,000
Gross proceeds available for issuance			9,500,000			9,500,000
At-The-Market (ATM) Equity Issuance Program | Other Expense
Dividends and Equity Issuance [Line Items]
Payments for stock issuance			$ 214,000		$ 195,000	$ 680,000
Capital gain
Dividends and Equity Issuance [Line Items]
Unrecaptured Section 1250 gain dividends per share					$ 0.305