Summary of Operating Results by Quarter (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 Financial Information [Line Items]
Interest income	$ 84,623	$ 84,979	$ 85,350	$ 84,733	$ 85,505	$ 85,624	$ 86,551	$ 85,973	$ 339,685	$ 343,653	$ 346,507
Interest expense	4,394	4,614	4,977	5,644	5,972	6,550	6,633	7,525	19,629	26,680	35,894
Net interest income	80,229	80,365	80,373	79,089	79,533	79,074	79,918	78,448	320,056	316,973	310,613
Provision for loan losses	4,000	4,500	4,500	4,500	5,000	4,500	5,600	7,100	(17,500)	(22,200)	(31,510)
Noninterest income	109,274	106,321	110,226	132,301	97,769	100,957	107,856	107,750	458,122	414,332	360,370
Noninterest expense	157,959	145,905	144,686	141,904	142,221	139,428	145,581	135,516	590,454	562,746	512,622
Income tax expense	6,484	10,156	12,248	18,619	6,815	10,088	10,272	12,712	(47,507)	(39,887)	(35,849)
Net income	$ 21,060	$ 26,125	$ 29,165	$ 46,367	$ 23,266	$ 26,015	$ 26,321	$ 30,870	$ 122,717	$ 106,472	$ 91,002
Net income-basic	$ 0.53	$ 0.65	$ 0.73	$ 1.16	$ 0.58	$ 0.65	$ 0.66	$ 0.77	$ 3.07	$ 2.66	$ 2.27
Net income-diluted	$ 0.52	$ 0.64	$ 0.72	$ 1.15	$ 0.58	$ 0.64	$ 0.65	$ 0.76	$ 3.04	$ 2.64	$ 2.26
Dividend	$ 0.215	$ 0.205	$ 0.205	$ 0.205	$ 0.205	$ 0.195	$ 0.195	$ 0.195	$ 0.83	$ 0.79	$ 0.75
Book value	$ 31.71	$ 31.88	$ 30.89	$ 29.90	$ 29.46	$ 28.97	$ 27.97	$ 26.62