Long-Term Incentive Plan (Vested Unit Awards) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	266,028	361,711	506,393
Fair Value of Unit Awards on Vesting Date (in millions)	$ 19.9	$ 22.6	$ 27.7
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Vested	$ 18.9	$ 27.4	$ 34.4