Debt - $700 Million Revolving Credit Facility - Additional Information (Detail) - $700 Million Revolving Credit Facility [Member]	12 Months Ended
	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Spread rate added to LIBOR	1.55%
Debt Instrument, description of interest rate	(i) LIBOR plus the applicable margin ranging from 1.55% to 2.40%, dependent upon the Company's funded debt to total asset value ratio (as defined in the Amended Credit Agreement) or (ii) a base rate as set in the Amended Credit Agreement.
Maturity date for credit facility	May 23, 2021
Additional borrowings	$ 0
Minimum [Member]
Debt Instrument [Line Items]
Spread rate added to LIBOR	1.55%
Maximum [Member]
Debt Instrument [Line Items]
Spread rate added to LIBOR	2.40%