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Fair Value Measurements - Additional Information (Detail) - USD ($)			9 Months Ended
	Jan. 17, 2017	Jul. 18, 2013	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Feb. 28, 2019	Jan. 31, 2019	Dec. 31, 2018
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%
Aggregate principal amount		$ 225,000,000
Debt issuance costs paid		5,039,000
Original debt issue discount		$ 1,125,000
Principal Repurchase Amount Of Exchangeable Notes							$ 250,000	$ 500,000
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%	5.00%
Aggregate principal amount	$ 86,250,000
Debt issuance costs paid	$ 5,902,000		$ 5,902,000
Principal Repurchase Amount Of Exchangeable Notes						$ 920,000		$ 1,590,000
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