Retirement Benefits - Fair Value Measurements (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Defined Benefit Plan Disclosure [Line Items] | |||
| Net asset value per share practical expedient(a) | $ 20.3 | $ 17.4 | |
| Non-U.S. Plans | |||
| Defined Benefit Plan Disclosure [Line Items] | |||
| Fair value of plan assets | 202.9 | 173.4 | $ 164.8 |
| Non-U.S. Plans | Level 1 | |||
| Defined Benefit Plan Disclosure [Line Items] | |||
| Fair value of plan assets | $ 182.6 | $ 156.0 | |