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Summary of Significant Accounting Policies - Narrative (Details) | 12 Months Ended
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Dec. 31, 2019 USD ($) segment subsidiary | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Property, Plant and Equipment |  |  |  |
Loans and Leases Receivable, Past Due Status, Policy [Policy Text Block] | ninety days |  |  |
Cash and Cash Equivalents | Cash and Cash Equivalents. For purposes of reporting cash flows, cash and cash equivalents include cash on hand, amounts due from banks, federal funds sold for one day periods and interest bearing deposits in banks with original maturities of less than three months. |  |  |
Finite-Lived Intangible Asset, Useful Life | 10 years |  |  |
Real Estate Acquired Through Foreclosure, Valuation Adjustments | $ 900,000 | $ 100,000 |  |
Number of significant subsidiaries | subsidiary | 1 |  |  |
Cash on deposit with Federal Reserve Bank | $ 769,300,000 | 574,400,000 |  |
Compensating balances with Federal Reserve Bank | 46,300,000 | 28,100,000 |  |
Deferred compensation plan assets | 18,200,000 | 12,100,000 |  |
Deferred compensation plan liabilities | 18,200,000 | 12,100,000 |  |
Impairment losses | 0 | 0 |  |
Write-downs of OREO | (900,000) | (100,000) | $ (400,000) |
Carrying value of foreclosed OREO | 2,300,000 | 2,000,000.0 |  |
Federal Reserve Bank stock | 42,800,000 | 37,500,000 |  |
Federal Home Loan Bank stock | 10,700,000 | 10,700,000 |  |
Income tax examination, penalties | $ 0 | 0 |  | $ 0
Number of operating segments | segment | 1 |  |  |
Advertising expense | $ 4,200,000 | 3,200,000 | 3,500,000 |
Stock compensation expense | 8,000,000.0 | 5,600,000 | 3,800,000 |
Stock-based compensation expense |  | (64,100,000) | (48,600,000) |
Mortgage Loans in Process of Foreclosure, Amount | $ 1,400,000 |  |  |
Benefit of stock-based compensation plans |  |  | (2,600,000) |
Short-term Leases [Policy Text Block] | 12 months |  |  |
Buildings and improvements | Minimum |  |  |  |
Property, Plant and Equipment |  |  |  |
Estimated useful lives | 5 years |  |  |
Buildings and improvements | Maximum [Member] |  |  |  |
Property, Plant and Equipment |  |  |  |
Estimated useful lives | 45 years |  |  |
Furniture and equipment | Minimum |  |  |  |
Property, Plant and Equipment |  |  |  |
Estimated useful lives | 4 years |  |  |
Furniture and equipment | Maximum [Member] |  |  |  |
Property, Plant and Equipment |  |  |  |
Estimated useful lives | 15 years |  |  |
Restricted Stock [Member] |  |  |  |
Property, Plant and Equipment |  |  |  |
Stock compensation expense |  | 5,600,000 | 3,300,000 |
Tax benefits from stock-based compensation | $ 400,000 | 200,000 | 600,000 |
Stock-based compensation expense |  | 0 |  |
Common Stock Including Additional Paid in Capital [Member] |  |  |  |
Property, Plant and Equipment |  |  |  |
Stock compensation expense | 8,000,000.0 | 5,600,000 | 3,800,000 |
Stock-based compensation expense |  |  | $ 0 |
Benefit of stock-based compensation plans | $ (1,200,000) | $ (1,100,000) |  |

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