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Accounting Policies and Basis of Presentation Accounting Policies and Basis of Presentation (Recently Adopted Accounting Standards) (Details) - USD ($) |  | 3 Months Ended | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 12, 2020 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Jun. 30, 2020 | Jan. 01, 2020 | Dec. 31, 2019 | Jun. 30, 2019 | Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Stock issued (in shares) |  |  |  | 339,937,000 |  |  |  |  |  |
Preferred stock, par value (in dollars per share) |  | $ 0.01 |  | $ 0.01 |  |  |  | $ 0.01 |  |
Proceeds from Issuance of Preferred Stock and Preference Stock |  |  |  | $ 345,000,000 | $ 0 |  |  |  |  |
Stock repurchased during period (in shares) |  | 117,700 | 295,900 | 813,090 | 744,400 |  |  |  |  |
Stock repurchased during period |  | $ 47,060,000 | $ 135,400,000 | $ 333,755,000 | $ 325,900,000 |  |  |  |  |
Stock repurchased during period (in USD per share) |  | $ 399.82 | $ 457.50 | $ 410.48 | $ 437.84 |  |  |  |  |
Forecast period (years) |  |  |  | 2 years |  |  |  |  |  |
Expected credit losses on SBA-PPP loans |  | $ 0 |  | $ 0 |  |  |  |  |  |
Reserve for unfunded commitments |  | 14,000,000.0 |  | 14,000,000.0 |  |  |  |  |  |
Retained earnings |  | 3,738,417,000 |  | 3,738,417,000 |  |  |  | $ 3,658,197,000 |  |
Allowance for credit loss |  | 223,936,000 | $ 226,825,000 | 223,936,000 | $ 226,825,000 | $ 222,450,000 |  | 225,141,000 | $ 226,583,000 | $ 223,712,000
Allowance for credit loss on debt securities held to maturity |  | 0 |  | 0 |  |  |  |  |  |
Loans and leases, not acquired in a transfer accounted for as debt securities |  | 32,349,266,000 |  | 32,349,266,000 |  |  |  | 28,322,780,000 |  |
Loans and leases |  | 336,382,000 | 315,012,000 | 988,029,000 | 909,167,000 |  |  |  |  |
Impairment loss |  | 0 |  |  |  |  |  |  |  |
Commercial |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Loans and leases, not acquired in a transfer accounted for as debt securities |  | 22,613,932,000 |  | 22,613,932,000 |  |  |  | $ 18,552,002,000 |  |
SBA-PPP | Commercial |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Loans and leases, not acquired in a transfer accounted for as debt securities |  | 3,112,676,000 |  | 3,112,676,000 |  |  |  |  |  |
Loans and leases |  | 28,900,000 |  | 47,900,000 |  |  |  |  |  |
Held-to-maturity Securities |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Expected allowance for credit loss |  | 0 |  | 0 |  |  |  |  |  |
Fixed-to-Floating Rate Subordinated Notes Due 2030 |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Debt instrument face amount |  | $ 350,000,000 |  | $ 350,000,000 |  |  |  |  |  |
Interest rate (percentage) |  | 3.375% |  | 3.375% |  |  |  |  |  |
Shares issued on conversion (in shares) |  | 0.025000 |  | 0.025000 |  |  |  |  |  |
Liquidation preference per share (in USD per share) |  | $ 25 |  | $ 25 |  |  |  |  |  |
Non-Cumulative Perpetual Preferred Stock, Series A |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Dividend rate (percentage) |  |  |  | 5.375% |  |  |  |  |  |
Preferred stock, par value (in dollars per share) |  | 0.01 |  | $ 0.01 |  |  |  |  |  |
Liquidation preference per share (in USD per share) |  | $ 1,000 |  | $ 1,000 |  |  |  |  |  |
Depositary Shares |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Stock issued (in shares) | 13,800,000 |  |  |  |  |  |  |  |  |
Class A Common Stock |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Stock repurchased during period |  |  | $ 135,386,000 |  | $ 325,916,000 |  |  |  |  |
Class A Common Stock | Repurchases From Ella Ann Holding |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Stock repurchased during period (in shares) |  |  |  | 45,000 |  |  |  |  |  |
Number of shares authorized to be repurchased (in shares) |  | 250,000 |  | 250,000 |  |  |  |  |  |
Cumulative effect of adoption of ASC 326 |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Reserve for unfunded commitments |  |  |  |  |  |  | $ 8,900,000 |  |  |
Retained earnings |  |  |  |  |  |  | 36,900,000 |  |  |
Allowance for credit loss |  |  |  |  |  |  | (37,900,000) |  |  |
Allowance for credit loss on debt securities held to maturity |  |  |  |  |  |  | 0 |  |  |
Cumulative effect of adoption of ASC 326 | Non-PCD Loans |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Allowance for credit loss |  |  |  |  |  |  | (56,900,000) |  |  |
Cumulative effect of adoption of ASC 326 | PCD Loans |  |  |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |  |  |
Allowance for credit loss |  |  |  |  |  |  | $ 19,000,000.0 |  |  |

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