Earnings per Share Earnings per Share - Table (Detail) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended				9 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Continuing operations
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest					$ 70,032	$ 29,100	$ 6,301	$ 77,044
Less: Net income attributable to noncontrolling interest					(1,218)	(112)	(2,078)	(1,531)
Consolidated net income (loss)					68,814		4,223	75,513
Less: Undistributed net income from continuing operations allocated to participating securities					0		0	(13,493)
Earnings from continuing operations available to common shareholders for basic earnings per share					68,814		4,223	62,020
Discontinued operations
Net income (loss) from discontinued operations available for diluted earnings per share	$ 7,191	$ 37,193	$ 1,229	$ 418	0	$ 38,840	46,031	(16,865)
Less: Undistributed net income from discontinued operations allocated to participating securities					0		0	3,014
Earnings (loss) from discontinued operations available to common shareholders for basic earnings per share					$ 0		$ 46,031	$ (13,851)
Denominator:
Weighted average common shares outstanding - basic					41,939	42,204	42,143	34,546
Weighted average common shares upon conversion of participating securities					0		0	7,515
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements					30		238	273
Weighted average common shares outstanding - diluted					41,969	42,512	42,381	42,334
Anti-dilutive awards excluded from diluted weighted average shares outstanding					0		1,093	2,102