Fair Value Measurements (Financial Instruments Measured At Fair Value On A Recurring Basis) (Details) (USD $) | Mar. 31, 2014 | Dec. 31, 2013 |
In Millions, unless otherwise specified |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Good Faith and Margin Deposits with Broker-Dealers | $21,658.50 | $21,355.10 |
Fair Value, Measurements, Recurring [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Marketable Securities | 72.2 | ' |
Investment Owned, at Fair Value | 524.9 | ' |
Total Assets at Fair Value | 597.1 | ' |
Contingent consideration at Fair Value | 19.2 | ' |
Total liabilities at fair value | 19.2 | ' |
Fair Value, Measurements, Recurring [Member] | US Treasury Securities [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Available-for-sale Securities at Fair Value | 19.2 | ' |
Fair Value, Measurements, Recurring [Member] | Asset Backed Securities [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Available-for-sale Securities at Fair Value | 0.4 | ' |
Fair Value, Measurements, Recurring [Member] | Exchange Traded Funds [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Trading Securities at Fair Value | 52.6 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Marketable Securities | 71.8 | ' |
Investment Owned, at Fair Value | 524.9 | ' |
Total Assets at Fair Value | 596.7 | ' |
Contingent consideration at Fair Value | 0 | ' |
Total liabilities at fair value | 0 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Treasury Securities [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Available-for-sale Securities at Fair Value | 19.2 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Asset Backed Securities [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Available-for-sale Securities at Fair Value | 0 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Exchange Traded Funds [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Trading Securities at Fair Value | 52.6 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Marketable Securities | 0.4 | ' |
Investment Owned, at Fair Value | 0 | ' |
Total Assets at Fair Value | 0.4 | ' |
Contingent consideration at Fair Value | 0 | ' |
Total liabilities at fair value | 0 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Treasury Securities [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Available-for-sale Securities at Fair Value | 0 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Asset Backed Securities [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Available-for-sale Securities at Fair Value | 0.4 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Exchange Traded Funds [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Trading Securities at Fair Value | 0 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Marketable Securities | 0 | ' |
Investment Owned, at Fair Value | 0 | ' |
Total Assets at Fair Value | 0 | ' |
Contingent consideration at Fair Value | 19.2 | 15.7 |
Total liabilities at fair value | 19.2 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | US Treasury Securities [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Available-for-sale Securities at Fair Value | 0 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Asset Backed Securities [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Available-for-sale Securities at Fair Value | 0 | ' |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Exchange Traded Funds [Member] | ' | ' |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | ' | ' |
Trading Securities at Fair Value | $0 | ' |