Selected Quarterly Consolidated Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Abstract]
Interest income	$ 236,115	$ 231,021	$ 226,789	$ 219,680	$ 211,432	$ 205,715	$ 202,431	$ 202,335	$ 913,605	$ 821,913	$ 760,040
Interest expense	31,183	30,117	29,002	27,016	26,173	25,518	25,526	26,183	117,318	103,400	95,415
Net interest income	204,932	200,904	197,787	192,664	185,259	180,197	176,905	176,152	796,287	718,513	664,625
Provision for loan and lease losses	13,000	10,150	7,250	10,500	12,500	14,250	14,000	15,600	40,900	56,350	49,300
Non-interest income		65,846	64,551	63,042	70,617	66,412	65,075	62,374
Non-interest expense	171,049	161,823	164,419	163,784	161,871	156,097	152,778	152,445	661,075	623,191	555,341
Income before income tax expense	86,922	94,777	90,669	81,422	81,505	76,262	75,202	70,481	353,790	303,450	297,761
Income tax expense	17,029	30,281	29,090	21,951	23,845	24,445	24,599	23,434	98,351	96,323	93,032
Net income	$ 69,893	64,496	61,579	59,471	57,660	51,817	50,603	47,047	255,439	207,127	204,729
Earnings applicable to common shareholders		$ 62,426	$ 59,485	$ 57,342	$ 55,501	$ 49,634	$ 48,398	$ 44,921	$ 247,255	$ 199,031	$ 196,018
Earnings per common share:
Earnings per common share, Basic (in dollars per share)	$ 0.74	$ 0.68	$ 0.65	$ 0.62	$ 0.61	$ 0.54	$ 0.53	$ 0.49	$ 2.68	$ 2.17	$ 2.15
Earnings per common share, Diluted (in dollars per share)	$ 0.73	$ 0.67	$ 0.64	$ 0.62	$ 0.60	$ 0.54	$ 0.53	$ 0.49	$ 2.67	$ 2.16	$ 2.13
Noninterest Income	$ 66,039								$ 259,478	$ 264,478	$ 237,777
Gain on sale of investment securities, net									0	414	609
Impairment loss on securities recognized in earnings									$ 126	$ 149	$ 110