Debt - Components of Long-term Debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Principal	$ 630,000	$ 617,500
Unamortized debt issuance debt costs	1,025	1,506
5.25% Senior Notes due March 15, 2019 [Member] | Senior Notes [Member]
Debt Instrument [Line Items]
Principal	500,000	500,000
Unamortized debt issuance debt costs	260	1,506
6% Related Party Term Loan [Member] | Medium-term Notes [Member]
Debt Instrument [Line Items]
Principal	130,000
Unamortized debt issuance debt costs	765
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Principal	$ 0	$ 117,500