Fair Value Measurements (Summary of Quantitative Information About Level 3 Fair Value Measurements) (Details)	12 Months Ended
	Dec. 31, 2017
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
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
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
Minimum [Member] | Auction Rate Securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Estimated Market Rate, Percent	3.10%
Minimum [Member] | Private Equity Funds [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
Minimum [Member] | Mortgage Loan Commitments [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Probability of Funding	50.40%
Embedded Servicing Value, Mortgage Loan Commitments, Discounted Cash Flow	(0.30%)
Maximum [Member] | Auction Rate Securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Estimated Market Rate, Percent	4.50%
Maximum [Member] | Private Equity Funds [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
Maximum [Member] | Mortgage Loan Commitments [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Probability of Funding	100.00%
Embedded Servicing Value, Mortgage Loan Commitments, Discounted Cash Flow	2.00%
Weighted Average [Member] | Mortgage Loan Commitments [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Weighted Average, Probability of Funding	76.00%
Weighted Average, Embedded Servicing Value	1.10%