Dividends and Equity Issuance Program - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended			51 Months Ended
	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Sep. 30, 2017
Dividends and Equity Issuance [Line Items]
Declared and paid dividends, per share		$ 2.64	$ 2.60	$ 2.56
Ordinary income per share		1.47	2.00	2.17
Capital gain per share		$ 1.17	$ 0.60	$ 0.39
Securities, aggregate sales price		$ 23,300,000			$ 23,300,000
At-The-Market Equity Issuance Program (ATM)
Dividends and Equity Issuance [Line Items]
Aggregate sales of threshold amount						$ 23,300,000
Share issued	249,016	127,499	249,016	0	957,415
Average sale price per share	$ 55.30	$ 74.71	$ 55.30		$ 52.22
Net cash proceeds from stock issued		$ 9,100,000	$ 13,200,000		$ 47,900,000
Payment of stock issuance cost		400,000	558,000		2,100,000
Payments for stock issuance		238,000	344,000		1,250,000
At-The-Market Equity Issuance Program (ATM) | Other Expense
Dividends and Equity Issuance [Line Items]
Payments for stock issuance		$ 162,000	$ 214,000		$ 840,000
Capital gain
Dividends and Equity Issuance [Line Items]
Unrecaptured Section 1250 gain dividends per share		$ 453