Schedule of Selected Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information [Line Items]
Revenue	$ 497,809	$ 508,522	$ 490,294	$ 484,064	$ 482,193	$ 462,728	$ 449,929	$ 440,916	$ 1,980,689	$ 1,835,766	$ 1,765,498
Operating income	66,251	71,095	70,954	73,264	64,649	64,419	61,712	58,705	281,564	249,485	260,396
Net income	$ 41,974	$ 48,994	$ 48,578	$ 49,340	$ 41,239	$ 40,994	$ 39,197	$ 37,777	$ 188,886	$ 159,207	$ 178,040
Basic earnings per share: Net income	$ 0.35	$ 0.41	$ 0.41	$ 0.42	$ 0.35	$ 0.35	$ 0.33	$ 0.32	$ 1.59	$ 1.34	$ 1.51
Diluted earnings per share: Net income	$ 0.35	$ 0.41	$ 0.41	$ 0.41	$ 0.35	$ 0.34	$ 0.33	$ 0.32	$ 1.59	$ 1.34	$ 1.50