Loans and Allowance for Credit Losses Loans Modified Under Troubled Debt Restructurings (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	$ 68,383	$ 55,150
Non-accrual TDRs (1)	31,575	20,825
Total TDRs	99,958	75,975
Loans and leases 90 days or more past due and still accruing	14,767	16,057
Financing Receivable, Nonaccrual	125,037	125,098
Financing Receivable, Not Past Due	18,514,573	16,620,575
Loans and Leases Receivable, Net of Deferred Income	18,704,722	16,837,526	$ 16,368,458
Real estate - commercial mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans and leases 90 days or more past due and still accruing	1,599	4,113
Financing Receivable, Nonaccrual	40,775	33,166
Financing Receivable, Not Past Due	6,874,941	6,651,042
Loans and Leases Receivable, Net of Deferred Income	6,934,936	6,700,776	6,497,973
Residential Mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	28,030	21,551
Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	20,407	13,330
Real-estate - home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	15,548	15,068
Loans and leases 90 days or more past due and still accruing	3,889	3,564
Financing Receivable, Nonaccrual	7,688	7,004
Financing Receivable, Not Past Due	1,234,854	1,292,035
Loans and Leases Receivable, Net of Deferred Income	1,251,455	1,314,944	$ 1,386,974
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	4,398	5,193
Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	0	8
Financial Asset, 30 to 59 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Past Due	39,485	56,326
Financial Asset, 30 to 59 Days Past Due | Real estate - commercial mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Past Due	16,431	10,912
Financial Asset, 30 to 59 Days Past Due | Real-estate - home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Past Due	3,431	9,635
Financial Asset, 60 to 89 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Past Due	10,860	19,470
Financial Asset, 60 to 89 Days Past Due | Real estate - commercial mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Past Due	1,190	1,543
Financial Asset, 60 to 89 Days Past Due | Real-estate - home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Past Due	$ 1,593	$ 2,706