Note 21 - Quarterly Financial Information (Detail) - Quarterly Financial Information (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
2012
Interest and fee income	$ 183,364	$ 207,979	$ 221,155
Net interest income	177,620	199,597	208,315
Provision for credit losses	11,200	11,200	11,200
Noninterest income	57,022	60,097	61,454
Noninterest expense	116,885	127,678	127,147
Income before taxes	106,557	120,816	131,422
Net income	81,127	87,888	94,577
Basic earnings per share (in Dollars per share)	$ 2.93	$ 3.07	$ 3.24
Diluted earnings per share (in Dollars per share)	$ 2.93	$ 3.06	$ 3.21
Dividends paid per share (in Dollars per share)	$ 1.48	$ 1.45	$ 1.44
Quarter Ending March 31 [Member]
2012
Interest and fee income	48,298	52,494	56,003
Net interest income	46,739	50,191	52,469
Provision for credit losses	2,800	2,800	2,800
Noninterest income	14,669	14,743	15,470
Noninterest expense	30,034	31,323	32,031
Income before taxes	28,574	30,811	33,108
Net income	21,005	22,382	23,576
Basic earnings per share (in Dollars per share)	$ 0.75	$ 0.77	$ 0.81
Diluted earnings per share (in Dollars per share)	$ 0.75	$ 0.77	$ 0.80
Dividends paid per share (in Dollars per share)	$ 0.37	$ 0.36	$ 0.36
Price range, common stock	43.90-49.53	49.25-56.96	50.87-61.25
Quarter Ending June 30 [Member]
2012
Interest and fee income	46,901	53,088	55,078
Net interest income	45,429	50,935	51,933
Provision for credit losses	2,800	2,800	2,800
Noninterest income	13,533	15,292	15,770
Noninterest expense	29,349	34,309	32,095
Income before taxes	26,813	29,118	32,808
Net income	20,964	21,269	23,561
Basic earnings per share (in Dollars per share)	$ 0.76	$ 0.74	$ 0.81
Diluted earnings per share (in Dollars per share)	$ 0.75	$ 0.74	$ 0.80
Dividends paid per share (in Dollars per share)	$ 0.37	$ 0.36	$ 0.36
Price range, common stock	43.01-48.62	46.91-52.53	52.17-60.37
Quarter Ending September 30 [Member]
2012
Interest and fee income	45,272	51,976	55,203
Net interest income	43,890	49,905	52,107
Provision for credit losses	2,800	2,800	2,800
Noninterest income	14,626	15,205	15,071
Noninterest expense	29,269	31,383	31,508
Income before taxes	26,447	30,927	32,870
Net income	20,022	22,432	23,709
Basic earnings per share (in Dollars per share)	$ 0.73	$ 0.79	$ 0.81
Diluted earnings per share (in Dollars per share)	$ 0.73	$ 0.79	$ 0.81
Dividends paid per share (in Dollars per share)	$ 0.37	$ 0.36	$ 0.36
Price range, common stock	44.08-49.39	36.32-50.52	50.04-55.99
Quarter Ending December 31 [Member]
2012
Interest and fee income	42,893	50,421	54,871
Net interest income	41,562	48,566	51,806
Provision for credit losses	2,800	2,800	2,800
Noninterest income	14,194	14,857	15,143
Noninterest expense	28,233	30,663	31,513
Income before taxes	24,723	29,960	32,636
Net income	$ 19,136	$ 21,805	$ 23,731
Basic earnings per share (in Dollars per share)	$ 0.70	$ 0.77	$ 0.82
Diluted earnings per share (in Dollars per share)	$ 0.70	$ 0.77	$ 0.81
Dividends paid per share (in Dollars per share)	$ 0.37	$ 0.37	$ 0.36
Price range, common stock	40.50-47.72	36.34-46.73	48.70-56.72