Business Segments (Operating Results and Other Key Financial Measures) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Net interest income before provision for credit losses	$ 368,219	$ 369,807	$ 367,326	$ 362,461	$ 369,416	$ 348,720	$ 341,679	$ 326,693	$ 1,467,813	$ 1,386,508	$ 1,185,069
Provision for credit losses	18,577	38,284	19,245	22,579	17,959	10,542	15,536	20,218	98,685	64,255	46,266
Noninterest income	63,013	51,474	52,759	42,131	41,695	46,502	48,268	74,444	209,377	210,909	257,748
Noninterest expense	193,373	176,630	177,663	186,922	188,097	179,815	177,419	169,135	734,588	714,466	661,451
INCOME BEFORE INCOME TAXES	219,282	206,367	223,177	195,091	205,055	204,865	196,992	211,784	843,917	818,696	735,100
Segment net income	188,215	$ 171,416	$ 150,380	$ 164,024	173,018	$ 171,302	$ 172,349	$ 187,032	674,035	703,701	505,624
Segment assets	44,196,096				41,042,356				44,196,096	41,042,356	37,121,563
Consumer and Business Banking
Segment Reporting Information
Net interest income before provision for credit losses									696,551	727,215	590,821
Provision for credit losses									14,178	9,364	1,812
Noninterest income									57,920	85,607	54,451
Noninterest expense									343,001	341,396	323,318
INCOME BEFORE INCOME TAXES									397,292	462,062	320,142
Segment net income									284,161	330,683	187,571
Segment assets	11,520,586				10,587,621				11,520,586	10,587,621	9,316,587
Commercial Banking
Segment Reporting Information
Net interest income before provision for credit losses									651,413	605,650	553,817
Provision for credit losses									84,507	54,891	44,454
Noninterest income									134,622	110,287	110,089
Noninterest expense									263,064	237,520	200,153
INCOME BEFORE INCOME TAXES									438,464	423,526	419,299
Segment net income									313,833	303,553	246,404
Segment assets	25,501,534				23,761,469				25,501,534	23,761,469	21,431,472
Other
Segment Reporting Information
Net interest income before provision for credit losses									119,849	53,643	40,431
Provision for credit losses									0	0	0
Noninterest income									16,835	15,015	93,208
Noninterest expense									128,523	135,550	137,980
INCOME BEFORE INCOME TAXES									8,161	(66,892)	(4,341)
Segment net income									76,041	69,465	71,649
Segment assets	$ 7,173,976				$ 6,693,266				$ 7,173,976	$ 6,693,266	$ 6,373,504