Income Taxes Computed at Federal Statutory Rate (Detail) - USD ($) $ in Thousands	3 Months Ended									9 Months Ended	12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income from continuing operations before taxes	$ (44,051)	$ (2,051)	$ 83,108	$ 39,537	$ (46,545)	$ (33,153)	$ 101,564	$ 3,288	$ 28,700	$ 34,400	$ 76,543	$ 25,154	$ 63,107
Provision for income taxes:
Tax at Federal statutory rate											16,074	5,282	17,266
Effect of the tax reform on deferred taxes													(11,101)
Impact of Company loss not subject to federal income taxes													53
State taxes net of federal benefit											5,224	1,626	1,864
Permanent differences											89	345	99
Change in valuation allowance net of effect of the tax reform											(113)	23	107
Other											(649)	241	(686)
Income tax expense											$ 20,625	$ 7,517	$ 7,602