Fair Value Measurements (Level 3 Significant Unobservable Inputs) (Details) - Fair Value, Measurements, Recurring - Level 3 - USD ($) $ / shares in Units, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Swaps Related to Sale of Certain Visa Class B Common Shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value swaps	$ 36.1	$ 29.7
Swaps Related to Sale of Certain Visa Class B Common Shares | Group One
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation Technique	Discounted Cash Flow	Discounted Cash Flow
Unobservable Inputs	Visa Class A Appreciation	Visa Class A Appreciation
Swaps Related to Sale of Certain Visa Class B Common Shares | Group Two
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Inputs	Conversion Rate	Conversion Rate
Swaps Related to Sale of Certain Visa Class B Common Shares | Group Three
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Inputs	Expected Duration	Expected Duration
Swaps Related to Sale of Certain Visa Class B Common Shares | Lower Limit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument visa class A appreciation (as percent)	7.00%	7.00%
Financial instrument conversion rate (as percent)	1.62%	1.63%
Financial instrument expected duration (in years)	1 year 3 months	1 year 6 months
Swaps Related to Sale of Certain Visa Class B Common Shares | Upper Limit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument visa class A appreciation (as percent)	11.00%	11.00%
Financial instrument conversion rate (as percent)	1.64%	1.65%
Financial instrument expected duration (in years)	3 years 9 months	4 years
Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value auction rate securities	$ 2.8	$ 4.3
Auction Rate | Group One
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation Technique	Comparables	Comparables
Unobservable Inputs	Price	Price
Auction Rate | Lower Limit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments fair value assumptions price per share (in usd per share)	$ 93	$ 92
Auction Rate | Upper Limit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments fair value assumptions price per share (in usd per share)	$ 100	$ 100