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Fair Value Measurements - Additional Information (Detail) - USD ($)			6 Months Ended		12 Months Ended
	Jan. 17, 2017	Jul. 18, 2013	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt issuance costs paid				$ 5,902,000
6.625% Senior Notes Due 2021 [Member] \| Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate		6.625%	6.625%		6.625%
Due date		Jul. 15, 2021	Jul. 15, 2021		Jul. 15, 2021
Aggregate principal amount		$ 225,000,000
Debt issuance costs paid		5,039,000
Original debt issue discount		$ 1,125,000
5.0% Exchangeable Senior Notes due March 2021 [Member] \| Exchangeable Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate	5.00%		5.00%		5.00%
Due date	Mar. 01, 2021		Mar. 01, 2021		Mar. 01, 2021
Aggregate principal amount	$ 86,250,000
Debt issuance costs paid	$ 5,902,000		$ 5,902,000
Other Asset Class [Member] \| Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets fair value on non-recurring basis			0	0
Liabilities fair value on non-recurring basis			$ 0	$ 0