Description of Business and Summary of Significant Accounting Policies - Income Per share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income (loss)									$ 176,100	$ 159,366	$ 111,511
Net income (loss)- Assuming dilution									$ 176,100	$ 159,366	$ 111,511
Net income (loss), shares									51,147	51,009	51,241
Effect of dilutive stock-based compensation									224	303	371
Net income (loss)- Assuming dilution, shares									51,371	51,312	51,612
Net income (loss), per share	$ 1.41	$ 0.47	$ 0.92	$ 0.64	$ 0.94	$ 0.66	$ 1.01	$ 0.52	$ 3.44	$ 3.12	$ 2.18
Net income (loss)- Assuming dilution, per share	$ 1.41	$ 0.46	$ 0.92	$ 0.63	$ 0.94	$ 0.66	$ 1.00	$ 0.51	$ 3.43	$ 3.11	$ 2.16