Fair Value Measurements (Level 3 Significant Unobservable Inputs) (Details) - Fair Value, Measurements, Recurring - Level 3 - Swaps Related to Sale of Certain Visa Class B Common Shares - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 35.0	$ 32.8
Group One
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Inputs	Visa Class A Appreciation	Visa Class A Appreciation
Group Two
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Inputs	Conversion Rate	Conversion Rate
Group Three
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable Inputs	Expected Duration	Expected Duration
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%	1.62%
Financial instrument expected duration (in years)	1 year 3 months	1 year 6 months
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%	1.64%
Financial instrument expected duration (in years)	3 years 9 months	4 years