Quarterly Operating Results (Unaudited) - Quarterly Operating Results (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 393,020	$ 421,418	$ 397,764	$ 363,594	$ 343,165	$ 359,310	$ 325,792	$ 335,012	$ 1,575,796	$ 1,363,279	$ 1,200,032
Total expenses	354,574	308,733	295,983	276,757	266,723	263,855	239,571	235,521	1,236,047	1,005,670	895,221
Income before income taxes	38,446	112,685	101,781	86,837	76,442	95,455	86,221	99,491	339,749	357,609	304,811
Net income	$ 24,395	$ 68,331	$ 61,755	$ 52,415	$ 47,225	$ 57,749	$ 52,007	$ 60,131	$ 206,896	$ 217,112	$ 184,045
Net income per share:
Basic	$ 0.17	$ 0.47	$ 0.43	$ 0.36	$ 0.32	$ 0.40	$ 0.36	$ 0.42	$ 1.43	$ 1.50	$ 1.28
Diluted	$ 0.17	$ 0.47	$ 0.42	$ 0.36	$ 0.32	$ 0.39	$ 0.36	$ 0.41	$ 1.41	$ 1.48	$ 1.26