Consolidated Statements of Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Thousands	9 Months Ended			12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	[1]	Sep. 30, 2018	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net derivative gains arising during the period, net of tax	$ 1,604	$ 6,356		$ (3,075)	$ 2,972
Reclassification of losses into net loss, net of tax	$ 3,108	$ 1,913		$ 3,005	$ 3,379

[1] The financial information presented above for the nine months ended September 30, 2016 is unaudited.