Long-Term Debt and Long-Term Debt Payable to Related Parties (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 400,000	$ 670,710
Promissory Notes	412,500	412,500
Total long-term debt	812,500	1,083,210
Converted Term Loan [Member]
Debt Instrument [Line Items]
Long-term debt	400,000	400,000
Term Loan [Member]
Debt Instrument [Line Items]
Long-term debt		270,710
2019 Promissory Notes [Member]
Debt Instrument [Line Items]
Promissory Notes	300,000	300,000
2042 Promissory Notes [Member]
Debt Instrument [Line Items]
Promissory Notes	$ 112,500	$ 112,500