Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2018	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (13,955)	$ (28,870)	$ (15,087)	$ (52,364)
Net derivative gains (losses) arising during the period, net of tax of $(3,075), $1,604, $6,356(1) and $2,972, respectively	(2,425)	(9,480)	6,820	(4,692)
Reclassification of losses into net loss, net of tax of $3,005, $3,108, $1,913(1) and $3,379, respectively	4,700	2,844	6,804	5,025
Other comprehensive income (loss)	2,275	(6,636)	13,624	333
Comprehensive loss	$ (11,680)	$ (35,506)	$ (1,463)	$ (52,031)