Income Taxes Computed at Federal Statutory Rate (Detail) - USD ($) $ in Thousands	3 Months Ended								9 Months Ended	12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Income from continuing operations before taxes	$ (36,925)	$ (11,421)	$ 82,783	$ 28,670	$ (29,092)	$ (21,766)	$ 66,996	$ 31,138	$ 34,400	$ 63,107	$ 47,276	$ 78,672
Provision for income taxes:
Tax at Federal statutory rate										17,266	16,546	27,535
Effect of the tax reform on deferred taxes										(11,101)
Impact of Partnership loss not subject to federal income taxes										53	741	477
State taxes net of federal benefit										1,864	3,170	5,672
Permanent differences										99	89	80
Change in valuation allowance net of effect of the tax reform										107	115	26
Other										(686)	(285)	(52)
Income tax expense										$ 7,602	$ 20,376	$ 33,738