Debt (Tables)

6 Months Ended

Jun. 30, 2013

Debt Disclosure [Abstract]

Carrying amount and fair value of debt

(Thousands of dollars)	June 30, 2013				June 30, 2012
	Carrying Amount		Fair Value		Carrying Amount		Fair Value
Publicly traded debt	$	1,630,197	$	1,637,505	$	632,448	$	694,626
Non-traded debt	4,386		4,245		7,387		7,120