Derivative Financial Instruments - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Jun. 20, 2016	Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Cash flow hedge ineffectiveness	Ineffectiveness was de minimis for the three months ended March 31, 2018 and 2017.
Unsecured term note	$ 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	277,000	$ 1,622,000
Derivative notional amount					$ 100,000,000	$ 50,000,000
Realized loss reclassified from accumulated other comprehensive loss to interest expense	(196,000)	$ (1,119,000)
Fair value of interest rate swap agreements, asset	277,000		205,000
Senior Term Note Due July 1, 2026 [Member]
Derivative [Line Items]
Unsecured term note	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	$ (30,000)		900,000
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
Interest Rate Swap [Member]
Derivative [Line Items]
Unsecured term note	100,000,000
Effective portion of gain (loss) on derivatives net of reclassification to interest expense	0		$ 0
Derivative notional amount	$ 100,000,000