Derivative Financial Instruments - Additional Information (Detail) - USD ($)		3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 20, 2016	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Cash flow hedge ineffectiveness				Ineffectiveness was de minimis for the three and six months ended June 30, 2018 and 2017.
Unsecured term note		$ 1,625,000,000		$ 1,625,000,000		$ 1,625,000,000
Repayments of variable rate term notes						225,000,000
Effective portion of gain (loss) on derivatives net of reclassification to interest expense		106,000	$ 237,000	383,000	$ 1,859,000
Derivative notional amount							$ 100,000,000	$ 50,000,000
Realized loss reclassified from accumulated other comprehensive loss to interest expense		(113,000)	(956,000)	(309,000)	(2,076,000)
Fair value of interest rate swap agreements, asset		177,000		177,000		205,000
Senior Term Note Due July 1, 2026 [Member]
Derivative [Line Items]
Unsecured term note		600,000,000		$ 600,000,000		600,000,000
Settlement of forward starting swap agreements	$ 9,200,000
Amortized period on interest expense				10 years
Interest Expense [Member]
Derivative [Line Items]
Realized loss reclassified from accumulated other comprehensive loss to interest expense		(100,000)	$ 700,000	$ (100,000)	$ 1,600,000
Unsecured Senior Notes [Member] | Senior Term Note Due July 1, 2026 [Member]
Derivative [Line Items]
Debt instrument principal amount	$ 600,000,000	600,000,000		600,000,000
Interest Rate Swap [Member]
Derivative [Line Items]
Unsecured term note		100,000,000		100,000,000
Effective portion of gain (loss) on derivatives net of reclassification to interest expense				0		$ 0
Derivative notional amount		$ 100,000,000		$ 100,000,000