Schedule of Financial Instruments Having Difference Between Carrying Amount and Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Note receivable from APM, Carrying Amount	$ 3,059	$ 2,920
Debt, Carrying Amount	(1,459,000)	(1,455,000)
Note receivable from APM, Fair Value	4,511	4,647
Debt, Fair Value	$ (1,490,063)	$ (1,459,625)