Quarterly Financial Data (Unaudited) (Narrative) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effect of Fourth Quarter Events [Line Items]
Revenues	$ 68,036	$ 62,749	$ 143,835	$ 45,440	$ 66,492	$ 53,638	$ 21,047	$ 103,124	$ 320,060	$ 244,301	$ 590,448
Net Income (Loss) Attributable to Parent	$ 22,336	$ 20,732	$ 60,439	$ 7,241	$ (10,336)	$ 13,907	$ 4,148	$ 55,887	$ 110,748	$ 63,606	$ 332,795
Net income per basic share (in Dollars per Share)	$ 0.15	$ 0.14	$ 0.39	$ 0.04	$ (0.06)	$ 0.08	$ 0.03	$ 0.34	$ 0.71	$ 0.39	$ 2.04
Net income per diluted share (in Dollars per Share)	$ 0.15	$ 0.14	$ 0.39	$ 0.04	$ (0.06)	$ 0.08	$ 0.03	$ 0.34	$ 0.71	$ 0.39	$ 2.03