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Fair Value Measurements (Summary of Quantitative Information About Level 3 Fair Value Measurements) (Details)
 12 Months Ended
Dec. 31, 2020
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Fair Value, Discounted Cash Flow, Valuation Techniques Discounted cash flow
Auction Rate Securities [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Fair Value, Discounted Cash Flow, Valuation Techniques Discounted cash flow
Fair Value, Estimated Market Recovery Period, Years 5 years
Auction Rate Securities [Member] | Minimum [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Fair Value, Estimated Market Rate, Percent 1.50%
Auction Rate Securities [Member] | Maximum [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Fair Value, Estimated Market Rate, Percent 1.70%
Auction Rate Securities [Member] | Weighted Average [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Fair Value, Estimated Market Recovery Period, Years 5 years [1]
Fair Value, Estimated Market Rate, Percent 1.50% [1]
Private Equity Funds [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Fair Value, Market Comparable Companies, Valuation Techniques Market comparable companies
Private Equity Funds [Member] | Minimum [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple 4.0
Private Equity Funds [Member] | Maximum [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple 6.0
Private Equity Funds [Member] | Weighted Average [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple 5.4 [1]
Mortgage Loan Commitments [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Fair Value, Discounted Cash Flow, Valuation Techniques Discounted cash flow
Mortgage Loan Commitments [Member] | Minimum [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Probability of Funding 56.70%
Embedded Servicing Value, Mortgage Loan Commitments, Discounted Cash Flow 0.50%
Mortgage Loan Commitments [Member] | Maximum [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Probability of Funding 98.80%
Embedded Servicing Value, Mortgage Loan Commitments, Discounted Cash Flow 1.00%
Mortgage Loan Commitments [Member] | Weighted Average [Member]  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]  
Probability of Funding 85.70% [1]
Embedded Servicing Value, Mortgage Loan Commitments, Discounted Cash Flow 0.90% [1]
[1] * Unobservable inputs were weighted by the relative fair value of the instruments.