Long-Term Incentive Plan (Total Non-Vested Unit Awards) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Long Term Incentive Plan [Line Items]
Non-vested units		417,172	396,114
Adjustment to Unit Awards in Anticipation of Achieving Above- Target Financial Results		43,861
Total Unit Award Accrual		461,033
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	[1]	$ 17.2
Performance Shares [Member]
Long Term Incentive Plan [Line Items]
Non-vested units		356,068	313,696
Time-Based Awards [Member]
Long Term Incentive Plan [Line Items]
Non-vested units		61,104	82,418
2016 Awards [Member] | Performance Shares [Member]
Long Term Incentive Plan [Line Items]
Non-vested units		175,445
Adjustment to Unit Awards in Anticipation of Achieving Above- Target Financial Results		43,861
Total Unit Award Accrual		219,306
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	[1]	$ 5.0
2017 Awards [Member] | Performance Shares [Member]
Long Term Incentive Plan [Line Items]
Non-vested units		180,623
Adjustment to Unit Awards in Anticipation of Achieving Above- Target Financial Results		0
Total Unit Award Accrual		180,623
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	[1]	$ 9.8
Retention Awards Vesting 2018 [Member] | Time-Based Awards [Member]
Long Term Incentive Plan [Line Items]
Non-vested units		31,174
Adjustment to Unit Awards in Anticipation of Achieving Above- Target Financial Results		0
Total Unit Award Accrual		31,174
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	[1]	$ 0.7
Retention Awards Vesting 2019 [Member] | Time-Based Awards [Member]
Long Term Incentive Plan [Line Items]
Non-vested units		28,214
Adjustment to Unit Awards in Anticipation of Achieving Above- Target Financial Results		0
Total Unit Award Accrual		28,214
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	[1]	$ 1.6
Retention Awards Vesting 2020 [Member] | Time-Based Awards [Member]
Long Term Incentive Plan [Line Items]
Non-vested units		1,716
Adjustment to Unit Awards in Anticipation of Achieving Above- Target Financial Results		0
Total Unit Award Accrual		1,716
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	[1]	$ 0.1

[1] Unrecognized compensation expense will be recognized over the remaining vesting period of the awards.