Fair Value Measurements (Summary Of Quantitative Information About Level 3 Fair Value Measurements) (Details)	6 Months Ended
	Jun. 30, 2018 Rate
Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Technique, Discounted cash flow	Discounted cash flow
Unobservable Input, Estimated market recovery period	5 years
Auction Rate Securities [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable Input, Estimated market rate	3.70%
Auction Rate Securities [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable Input, Estimated market rate	5.90%
Private Equity Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Technique, Market comparable companies	Market comparable companies
Private Equity Investments [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple	4.0
Private Equity Investments [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Earnings before Interest, Taxes, Depreciation, and Amortization Multiple	6.0
Mortgage Loan Commitments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Technique, Discounted cash flow	Discounted cash flow
Weighted Average, Probability of Funding	80.40%
Weighted Average, Embedded Servicing Value	1.30%
Mortgage Loan Commitments [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Probability of Funding	50.50%
Embedded Servicing Value	0.50%
Mortgage Loan Commitments [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Probability of Funding	98.80%
Embedded Servicing Value	2.40%