Loans and Allowance for Credit Losses Loans Modified Under Troubled Debt Restructurings (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	$ 48,412	$ 55,150
Non-accrual TDRs (1)	27,600	20,825
Total TDRs	76,012	75,975
Loans and leases 90 days or more past due and still accruing	19,593	16,057
Financing Receivable, Nonaccrual	120,345	125,098
Financing Receivable, Not Past Due	16,866,683	16,620,575
Loans and Leases Receivable, Net of Deferred Income	17,077,403	16,837,526	$ 16,262,633
Real-estate - commercial mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans and leases 90 days or more past due and still accruing	879	4,113
Financing Receivable, Nonaccrual	35,657	33,166
Financing Receivable, Not Past Due	6,837,453	6,651,042
Loans and Leases Receivable, Net of Deferred Income	6,895,069	6,700,776	6,428,688
Residential Mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	21,473	21,551
Commercial Real Estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	7,648	13,330
Real-estate - home equity
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	14,513	15,068
Loans and leases 90 days or more past due and still accruing	3,525	3,564
Financing Receivable, Nonaccrual	7,261	7,004
Financing Receivable, Not Past Due	1,273,110	1,292,035
Loans and Leases Receivable, Net of Deferred Income	1,292,677	1,314,944	$ 1,413,500
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	4,771	5,193
Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Accruing Troubled Debt Restructurings	7	8
Financial Asset, 30 to 59 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Past Due	55,773	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	20,052	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	6,782	9,635
Financial Asset, 60 to 89 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing Receivable, Past Due	15,009	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	1,028	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	$ 1,999	$ 2,706