Long-Term Incentive Plan (Weighted-Average Grant Date Fair Values) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Long Term Incentive Plan [Line Items]
Units granted	220,148
Weighted-Average Grant Date Fair Value	$ 81.89
Performance Shares [Member]
Long Term Incentive Plan [Line Items]
Units granted	189,544	193,344	148,028
Weighted-Average Grant Date Fair Value	$ 82.34	$ 70.29	$ 88.78
Time-Based Awards [Member]
Long Term Incentive Plan [Line Items]
Units granted	30,604	39,301	26,421
Weighted-Average Grant Date Fair Value	$ 79.10	$ 64.76	$ 81.51