Common Stock - Additional Information (Detail) - USD ($)		3 Months Ended	6 Months Ended		12 Months Ended
	Mar. 29, 2017	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Value of common stock	$ 150,000,000
Net proceeds to company			$ 69,139,000	$ (96,000)	$ 110,390,000	$ (111,000)
Common stock issued through Restricted Stock/Unit Plan, and Management Incentive Plan			77,000
Common stock issued, amount			$ 69,922,000
Dividend Reinvestment And Stock Purchase Plan [Member]
Common stock issued through Restricted Stock/Unit Plan, and Management Incentive Plan			73,000
Common stock issued, amount			$ 5,000,000
BNY Mellon Capital Markets, LLC [Member]
Common stock sold through agent		748,932	886,232		1,391,939
Open market weighted average price per share		$ 74.10	$ 73.00		$ 76.49
Net proceeds to company		$ 54,940,503	$ 64,048,167		$ 105,407,194
Agent commissions		554,954	646,951		1,064,719
Common stock available for sale under the program		$ 43,528,087	$ 43,528,087		$ 43,528,087