Long-Term Incentive Plan (Vested Unit Awards) (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Disclosure of Compensation Related Costs, Share-based Payments [Abstract] | |||
| Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Total Unit Award Accrual Number (in shares) | 317,037 | 266,028 | 361,711 |
| Fair Value of Unit Awards on Vesting Date (in millions) | $ 22.1 | $ 19.9 | $ 22.6 |
| Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Vested | $ 18.1 | $ 18.9 | $ 27.4 |