Earnings Per Share - Summary of Earnings Per Share, Basic and Diluted (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	$ 282,697	$ 202,823	$ 130,987	$ 123,693	$ 95,424	$ 129,618	$ 116,253	$ 111,090	$ 740,200	$ 452,385	$ 362,431
Denominator for basic earnings per share (in shares)	89,676	90,751	92,013	92,108	92,574	92,631	92,665	92,516	91,129	92,597	92,023
Dilutive securities (in shares)									2,465	2,616	2,116
Denominator for diluted earnings per share (in shares)	92,623	93,001	94,223	94,560	95,235	95,307	95,279	95,030	93,594	95,213	94,139
Basic earnings per share (in usd per share)	$ 3.15	$ 2.23	$ 1.42	$ 1.34	$ 1.03	$ 1.40	$ 1.25	$ 1.20	$ 8.12	$ 4.89	$ 3.94
Diluted earnings per share (in usd per share)	$ 3.05	$ 2.18	$ 1.39	$ 1.31	$ 1.00	$ 1.36	$ 1.22	$ 1.17	$ 7.91	$ 4.75	$ 3.85