Schedule of Selected Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information [Line Items]
Revenue	$ 482,193	$ 462,728	$ 449,929	$ 440,916	$ 440,576	$ 442,845	$ 436,393	$ 445,684	$ 1,835,766	$ 1,765,498	$ 1,849,785
Operating income	64,649	64,419	61,712	58,705	65,263	60,815	65,279	69,039	249,485	260,396	296,416
Net income	$ 41,239	$ 40,994	$ 39,197	$ 37,777	$ 41,340	$ 41,178	$ 45,475	$ 50,047	$ 159,207	$ 178,040	$ 219,919
Basic earnings per share: Net income	$ 0.35	$ 0.35	$ 0.33	$ 0.32	$ 0.35	$ 0.35	$ 0.38	$ 0.42	$ 1.34	$ 1.51	$ 1.87
Diluted earnings per share: Net income	$ 0.35	$ 0.34	$ 0.33	$ 0.32	$ 0.35	$ 0.35	$ 0.38	$ 0.42	$ 1.34	$ 1.50	$ 1.87