Business Segments (Operating Results and Other Key Financial Measures) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information
Net interest income before provision for credit losses	$ 1,377,193	$ 1,467,813	$ 1,386,508
Provision for credit losses	210,653	98,685	64,255
Noninterest income	235,547	222,245	217,433
Noninterest expense	716,322	747,456	720,990
Segment income before income taxes	685,765	843,917	818,696
Segment net income	567,797	674,035	703,701
Segment assets	52,156,913	44,196,096	41,042,356
Consumer and Business Banking
Segment Reporting Information
Net interest income before provision for credit losses	530,829	696,551	727,215
Provision for credit losses	3,885	14,178	9,364
Noninterest income	67,115	57,920	85,607
Noninterest expense	331,750	343,001	341,396
Segment income before income taxes	262,309	397,292	462,062
Segment net income	187,931	284,161	330,683
Segment assets	13,351,060	11,520,586	10,587,621
Commercial Banking
Segment Reporting Information
Net interest income before provision for credit losses	706,286	651,413	605,650
Provision for credit losses	206,768	84,507	54,891
Noninterest income	139,365	134,622	110,287
Noninterest expense	266,923	263,064	237,520
Segment income before income taxes	371,960	438,464	423,526
Segment net income	266,342	313,833	303,553
Segment assets	26,958,766	25,501,534	23,761,469
Other
Segment Reporting Information
Net interest income before provision for credit losses	140,078	119,849	53,643
Provision for credit losses	0	0	0
Noninterest income	29,067	29,703	21,539
Noninterest expense	117,649	141,391	142,074
Segment income before income taxes	51,496	8,161	(66,892)
Segment net income	113,524	76,041	69,465
Segment assets	$ 11,847,087	$ 7,173,976	$ 6,693,266