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 | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI loans	10,781,587
Commercial real estate | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI loans	77,884
Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI loans	42,560
Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI loans	862
Commercial real estate | Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI loans	1,495,717
Commercial real estate | Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI loans	1,487,877
Commercial real estate | Construction | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI loans	7,486
Commercial real estate | Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI loans	354
Commercial real estate | Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable recorded investment non-PCI loans	$ 0