Stock-Based Compensation - Summary of Restricted Stock Award Activity (Detail) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Change In Estimated Payout of Performance Unit Awards	20	(95)
Vested	(180)	(15)	(117)
Weighted Avg. Fair Value Per Share
Change In Estimated Payout of Performance Unit Awards, Weighted Average Fair Value Per Share	$ 21.84	$ 27.52
Vested	$ 11.65	$ 34.32	$ 37.25
Restricted Stock
Number of Shares
Restricted stock awards at Beginning of Year	820	726	590
Granted during the period	615	537	479
Cancellations during the period	0	0	(104)
Vested	(587)	(348)	(239)
Outstanding, at End of Year	868	820	726
Weighted Avg. Fair Value Per Share
Restricted stock awards at Beginning of Year	$ 17.72	$ 30.12	$ 37.13
Granted during the period	5.57	6.44	21.40
Cancellations during the period	0.00	0.00	21.84
Vested	15.39	23.50	33.60
Outstanding, at End of Year	$ 10.76	$ 17.72	$ 30.12
Phantom Share Units (PSUs)
Number of Shares
Restricted stock awards at Beginning of Year	1,053	82	153
Granted during the period	919	991	47
Cancellations during the period	(4)	(5)	(1)
Outstanding, at End of Year	1,788	1,053	82
Weighted Avg. Fair Value Per Share
Restricted stock awards at Beginning of Year	$ 7.60	$ 30.61	$ 38.43
Granted during the period	6.68	6.14	21.84
Cancellations during the period	15.31	19.05	30.87
Outstanding, at End of Year	$ 6.70	$ 7.60	$ 30.61