Business Segments - Selected Business Segment Financial Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business segment financial information
Net Interest Income	$ 496,322	$ 497,715	$ 486,352
Provision for Credit Losses	117,800	16,000	13,425
Net Interest Income (Loss) After Provision for Credit Losses	378,522	481,715	472,927
Noninterest Income	184,409	183,338	168,923
Noninterest Expense	(373,807)	(379,227)	(371,624)
Income (Loss) Before Provision for Income Taxes	189,124	285,826	270,226
Provision for Income Taxes	(35,320)	(59,913)	(50,624)
Net Income	153,804	225,913	219,602
Total Assets	20,603,651	18,095,496	17,143,974
Consumer Banking
Business segment financial information
Net Interest Income	292,710	305,803	305,681
Provision for Credit Losses	8,087	11,685	14,837
Net Interest Income (Loss) After Provision for Credit Losses	284,623	294,118	290,844
Noninterest Income	128,400	142,378	134,342
Noninterest Expense	(289,177)	(281,662)	(277,608)
Income (Loss) Before Provision for Income Taxes	123,846	154,834	147,578
Provision for Income Taxes	(31,476)	(38,654)	(37,285)
Net Income	92,370	116,180	110,293
Total Assets	7,478,813	7,054,511	6,715,095
Commercial Banking
Business segment financial information
Net Interest Income	188,626	185,259	179,577
Provision for Credit Losses	(948)	976	(760)
Net Interest Income (Loss) After Provision for Credit Losses	189,574	184,283	180,337
Noninterest Income	34,523	33,362	23,733
Noninterest Expense	(63,294)	(84,616)	(81,344)
Income (Loss) Before Provision for Income Taxes	160,803	133,029	122,726
Provision for Income Taxes	(40,081)	(28,852)	(28,496)
Net Income	120,722	104,177	94,230
Total Assets	5,116,807	4,254,261	3,958,523
Treasury and Other
Business segment financial information
Net Interest Income	14,986	6,653	1,094
Provision for Credit Losses	110,661	3,339	(652)
Net Interest Income (Loss) After Provision for Credit Losses	(95,675)	3,314	1,746
Noninterest Income	21,486	7,598	10,848
Noninterest Expense	(21,336)	(12,949)	(12,672)
Income (Loss) Before Provision for Income Taxes	(95,525)	(2,037)	(78)
Provision for Income Taxes	36,237	7,593	15,157
Net Income	(59,288)	5,556	15,079
Total Assets	$ 8,008,031	$ 6,786,724	$ 6,470,356