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Revolving Credit Facilities - Additional Information (Detail) - Unsecured Debt [Member]
12 Months Ended
Dec. 31, 2017
USD ($)
Revolving Credit Facility [Member]  
Debt Instrument [Line Items]  
Covenant compliance The Company was in compliance with these financial covenants at December 31, 2017 and 2016.
Revolving Credit Facility [Member] | LIBOR [Member]  
Debt Instrument [Line Items]  
Specified spread line of credit facility 1.20%
Revolving Credit Facility [Member] | Alternative Base Rate [Member]  
Debt Instrument [Line Items]  
Specified spread line of credit facility 0.20%
J.P. Morgan Chase Bank, N.A., Wells Fargo Securities, LLC, Citizens Bank, N.A., RBC Capital Markets and U.S. Bank National Association [Member] | Revolving Credit Facility [Member]  
Debt Instrument [Line Items]  
Unsecured revolving credit facility borrowing capacity $ 750,000,000
Accordion feature $ 1,250,000,000
Line of credit facility competitive bid option on periodic interest rates up to 50% of the facility
Facility fee 0.25%
J.P. Morgan Chase Bank, N.A., Wells Fargo Securities, LLC, Citizens Bank, N.A., RBC Capital Markets and U.S. Bank National Association [Member] | Amended Revolving Credit Facility [Member]  
Debt Instrument [Line Items]  
Lines of Credit Maturity Date 2021-09
Revolving credit facility maturity extension option two six-month options to extend the maturity to September 2022 upon the Company’s request
J.P. Morgan Chase Bank, N.A., Wells Fargo Securities, LLC, Citizens Bank, N.A., RBC Capital Markets and U.S. Bank National Association [Member] | Maximum [Member] | Amended Revolving Credit Facility [Member]  
Debt Instrument [Line Items]  
Unsecured revolving credit facility borrowing capacity $ 950,000,000
Accordion feature $ 1,450,000,000
PNC Bank National Association [Member] | Revolving Credit Facility [Member]  
Debt Instrument [Line Items]  
Lines of Credit Maturity Date 2021-09
PNC Bank National Association [Member] | Amended Revolving Credit Facility [Member]  
Debt Instrument [Line Items]  
Unsecured revolving credit facility $ 50,000,000