Selected Quarterly Consolidated Financial Information (Unaudited) (Details) - 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
Selected Quarterly Financial Information [Abstract]
Interest income	$ 198,160	$ 191,998	$ 186,970	$ 182,912	$ 183,751	$ 179,914	$ 177,497	$ 177,779	$ 760,040	$ 718,941	$ 687,640
Interest expense	24,820	23,988	23,459	23,148	23,103	22,544	22,375	22,478	95,415	90,500	90,912
Net interest income	173,340	168,010	163,511	159,764	160,648	157,370	155,122	155,301	664,625	628,441	596,728
Provision for loan and lease losses	13,800	13,000	12,750	9,750	9,500	9,500	9,250	9,000	49,300	37,250	33,500
Non-interest income (less securities amounts)	60,297	61,537	59,365	57,847	53,553	50,952	47,669	45,580
Gain on sale of investment securities, net	80	0	486	43	1,121	42	0	4,336	609	5,499	712
Impairment loss recognized in earnings	(28)	(82)	0	0	(899)	(85)	(73)	(88)	(110)	(1,145)	(7,277)
Non-interest expense	143,164	139,854	137,446	134,090	130,164	124,498	122,475	124,463	554,554	501,600	497,709
Income before income tax expense	76,725	76,611	73,166	73,814	74,759	74,281	70,993	71,666	300,316	291,699	256,569
Income tax expense	24,146	25,075	20,663	24,092	23,753	23,824	23,159	21,237	93,976	91,973	77,113
Net income	52,579	51,536	52,503	49,722	51,006	50,457	47,834	50,429	206,340	199,726	179,456
Earnings applicable to common shareholders	$ 50,414	$ 49,341	$ 50,277	$ 46,937	$ 48,224	$ 47,647	$ 45,008	$ 47,617	$ 197,629	$ 189,170	$ 168,653
Earnings per common share:
Earnings per common share, Basic (in dollars per share)	$ 0.55	$ 0.54	$ 0.55	$ 0.52	$ 0.54	$ 0.53	$ 0.50	$ 0.53	$ 2.17	$ 2.10	$ 1.90
Earnings per common share, Diluted (in dollars per share)	$ 0.55	$ 0.54	$ 0.55	$ 0.52	$ 0.53	$ 0.53	$ 0.50	$ 0.53	$ 2.15	$ 2.08	$ 1.86