Quarterly Financial Information (Unaudited) - Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 312,120	$ 252,820	$ 274,036	$ 253,148	$ 291,612	$ 245,015	$ 257,913	$ 246,451	$ 1,092,124	$ 1,040,991	$ 954,562
Depreciation and amortization	28,916	28,498	28,399	28,570	28,010	28,033	28,232	28,003	114,383	112,278	116,528
Operating income	36,389	32,768	57,015	35,890	43,739	29,083	47,486	32,797	162,062	153,105	76,188
Income before income taxes	30,469	22,079	42,255	4,853	61,582	14,667	28,567	20,169	99,656	124,985	25,690
(Provision) benefit for income taxes	8,430	4,612	(866)	(321)	1,096	463	(576)	484	11,855	1,467	92,662
Net income	38,899	26,691	41,389	4,532	62,678	15,130	27,991	20,653	111,511	126,452	118,352
Net income available to common stockholders	$ 38,899	$ 26,691	$ 41,389	$ 4,532	$ 62,213	$ 15,130	$ 23,039	$ 20,653	$ 111,511	$ 121,035	$ 113,483
Net income per share	$ 0.76	$ 0.52	$ 0.81	$ 0.09	$ 1.22	$ 0.30	$ 0.45	$ 0.41	$ 2.18	$ 2.38	$ 2.22
Net income per share - assuming dilution	$ 0.75	$ 0.52	$ 0.80	$ 0.09	$ 1.21	$ 0.25	$ 0.38	$ 0.32	$ 2.16	$ 2.17	$ 1.81