Loans and Allowance for Credit Losses for Loans - Credit Exposure by Internally Assigned Risk Rating by Class of Loans (Excluding PCI Loans) (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	$ 16,542,593
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	16,251,917
Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	119,088
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	109,425
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	62,163
Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	4,143,983
Commercial and industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	3,982,453
Commercial and industrial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	33,718
Commercial and industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	66,511
Commercial and industrial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	61,301
Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	10,902,893
Commercial real estate | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	1,495,717
Commercial real estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	10,781,587
Commercial real estate | Pass | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	1,487,877
Commercial real estate | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	77,884
Commercial real estate | Special Mention | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	7,486
Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	42,560
Commercial real estate | Substandard | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	354
Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	862
Commercial real estate | Doubtful | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI Loans	$ 0