Summary of Significant Accounting Policies Earnings per Unit (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Basic weighted average units outstanding (in shares)	56,212	56,061	55,933
Effect of dilutive units:
Deferred units (in shares)	48	42	31
Performance units (in shares)	135	188	181
Restricted units (in shares)	312	324	288
Unit options (in shares)	153	185	129
Diluted weighted average units outstanding (in shares)	56,860	56,800	56,562
Net income (loss) per unit - basic (in dollars per share)	$ 2.25	$ 3.84	$ 3.18
Net income (loss) per unit - diluted (in dollars per share)	$ 2.23	$ 3.79	$ 3.14