Quarterly Operating Results (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Results Of Operations [Line Items]
Total revenues	$ 302,830	$ 303,800	$ 290,916	$ 302,486	$ 244,097	$ 260,401	$ 246,816	$ 262,228	$ 1,200,032	$ 1,013,542	$ 973,492
Total expenses	233,603	222,151	219,771	219,696	184,406	187,709	185,348	185,558	895,221	743,021	707,394
Income before income taxes	69,227	81,649	71,145	82,790	59,691	72,692	61,468	76,670	304,811	270,521	266,098
Net income	$ 42,637	$ 49,504	$ 42,471	$ 49,433	$ 36,494	$ 44,173	$ 37,035	$ 46,293	$ 184,045	$ 163,995	$ 161,752
Net income per share:
Basic	$ 0.30	$ 0.34	$ 0.30	$ 0.34	$ 0.25	$ 0.31	$ 0.26	$ 0.32	$ 1.28	$ 1.15	$ 1.14
Diluted	$ 0.29	$ 0.34	$ 0.29	$ 0.34	$ 0.25	$ 0.30	$ 0.26	$ 0.32	$ 1.26	$ 1.13	$ 1.12