Commitments and Contingencies (Narrative) (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) Officers $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Employee Separation Expenses [Line Items]
Unaccrued contractual and other commitments	$ 117,900
Number Of Officers In Employment Contract | Officers	2
Aggregate base compensation	$ 1,300
Provision For Employee Separation Expenses	1,139	$ 1,299	$ 1,355
Employee Termination Expenses	1,100	$ 1,300	$ 1,400
Severance Costs	1,100
Environmental insurance coverage claims per occurrence	25,000
Environmental insurance coverage claims aggregate	$ 25,000
Restricted Stock [Member]
Employee Separation Expenses [Line Items]
RSUs granted | shares	461,395	336,296	264,989
Grant date fair value (in dollars per share) | $ / shares	$ 11.02	$ 14.95	$ 19.27
Officer [Member]
Employee Separation Expenses [Line Items]
Renewal of agreements term	1 year