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Income Taxes - Additional Information (Details) - USD ($)
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Income Taxes [Line Items]      
Net capital loss carryforwards $ 12,000,000    
Valuation allowances 12,000,000 $ 11,000,000  
Deferred tax asset 2,696,000,000 2,984,000,000  
Unrecognized tax benefits 45,000,000 40,000,000 $ 50,000,000
Net interest expense (benefit) (4,000,000) (1,500,000) (100,000)
Recovery of penalties related to unrecognized tax benefits in income tax expense 0 0 $ 0
Accrued interest payable 600,000 0  
Reduction in federal tax credit carryforward 0 $ 0  
First Niagara Bank, N.A.      
Income Taxes [Line Items]      
Allocated bad debt deductions for which no income taxes have been recorded 92,000,000    
Federal      
Income Taxes [Line Items]      
Operating loss carryforwards 7,000,000    
Credit carryforward 2,000,000    
State      
Income Taxes [Line Items]      
Operating loss carryforwards 23,000,000    
Deferred tax asset $ 1,000,000