Schedule of Selected Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information [Line Items]
Revenue	$ 440,576	$ 442,845	$ 436,393	$ 445,684	$ 464,134	$ 474,935	$ 463,331	$ 447,385	$ 1,765,498	$ 1,849,785	$ 1,793,087
Operating income	65,263	60,815	65,279	69,039	80,359	73,953	77,176	64,928	260,396	296,416	280,554
Net income	$ 41,340	$ 41,178	$ 45,475	$ 50,047	$ 60,689	$ 55,340	$ 57,583	$ 46,307	$ 178,040	$ 219,919	$ 221,854
Basic earnings per share: Net income	$ 0.35	$ 0.35	$ 0.38	$ 0.42	$ 0.52	$ 0.47	$ 0.49	$ 0.39	$ 1.51	$ 1.87	$ 1.90
Diluted earnings per share: Net income	$ 0.35	$ 0.35	$ 0.38	$ 0.42	$ 0.52	$ 0.47	$ 0.49	$ 0.39	$ 1.50	$ 1.87	$ 1.88