Basic net income (loss) per common unit is determined pursuant to the two-class method. Under this method, all earnings are allocated to our limited partners and participating securities based on their respective rights to receive distributions earned during the period. Participating securities include restricted units awarded under our long-term incentive plans. We compute basic net income (loss) per common unit by dividing net income (loss) attributable to common units by the weighted-average number of common units outstanding during the period.

We compute diluted net income (loss) per common unit by dividing net income (loss) attributable to common units by the sum of (i) the weighted average number of common units outstanding during the period and (ii) the effect of dilutive potential common units outstanding during the period. Dilutive potential common units may include contingently issuable performance unit awards and the Series D Preferred Units.

The Series D Preferred Units are convertible into common units at the option of the holder at any time on or after June 29, 2020. As such, we calculated the dilutive effect of the Series D Preferred Units using the if-converted method. The effect of the assumed conversion of the Series D Preferred Units outstanding as of the end of each period presented was antidilutive; therefore, we did not include such conversion in the computation of diluted net income (loss) per common unit.

Contingently issuable performance units are included as dilutive potential common units if it is probable that that performance measures will be achieved, unless to do so would be antidilutive. There were no dilutive potential common units outstanding related to the performance units for all periods presented.


	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
	(Thousands of Dollars, Except Unit and Per Unit Data)
Net income (loss)	$	63,383	$	29,766	$	105,640	$	(117,875)
Distributions to preferred limited partners	(31,887)		(30,684)		(63,774)		(61,107)
Distributions to common limited partners	(43,814)		(43,678)		(87,648)		(87,408)
Distribution equivalent rights to restricted units	(586)		(502)		(1,184)		(1,008)
Distributions in excess of income (loss)	$	(12,904)	$	(45,098)	$	(46,966)	$	(267,398)

Distributions to common limited partners	$	43,814	$	43,678	$	87,648	$	87,408
Allocation of distributions in excess of income (loss)	(12,904)		(45,098)		(46,966)		(267,398)
Series D Preferred Unit accretion	(4,155)		(5,064)		(8,176)		(9,966)
Net income (loss) attributable to common units	$	26,755	$	(6,484)	$	32,506	$	(189,956)

Basic and diluted weighted-average common units outstanding	109,529,658		109,194,722		109,518,004		109,046,061

Basic and diluted net income (loss) per common unit	$	0.25	$	(0.06)	$	0.30	$	(1.74)