Earnings Per Common Share (Narrative) (Details) - USD ($) $ in Millions |
9 Months Ended | |||
|---|---|---|---|---|
Dec. 14, 2015 |
Dec. 11, 2015 |
Sep. 30, 2016 |
Sep. 30, 2015 |
|
| Employee Stock Option [Member] | ||||
| Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] | ||||
| Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount | 143,191 | 185,123 | ||
| Interactive Data Holdings Corporation Acquisition [Member] | ||||
| Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] | ||||
| Business Combination, Consideration Transferred, Equity Interests Issued and Issuable | $ 6.5 | |||
| TRAYPORT [Member] | ||||
| Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] | ||||
| Business Combination, Consideration Transferred, Equity Interests Issued and Issuable | $ 2.5 | |||