Reconciliation of Our Investment in Restricted Equity Securities Measured at Fair Value on Quarterly Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Jan. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment in restricted equity securities	$ 56,400		$ 0
Balance at March 31, 2019	56,400		0
Restricted equity securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment in restricted equity securities	56,400		0
Balance at March 31, 2019	56,400		$ 0
Restricted equity securities | Fair Value Measurements Recurring | Significant Unobservable Inputs (Level 3) | Long-term investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment in restricted equity securities	56,400	$ 54,700
Unrealized gain recognized on restricted equity securities still held at March 31, 2019	1,700
Balance at March 31, 2019	$ 56,400	$ 54,700