Note 11 - Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Schedule of Asset Retirement Obligations [Table Text Block]

The following table provides asset retirement obligation activity (in millions):

	For the Year Ended December 31,
	2018			2017
Asset Retirement Obligations at start of period	$	43		$	38
Liabilities incurred in the current period		3			2
Liabilities settled in the current period		(3	)		(1	)
Accretion expense		2			1
Revision in estimated cash flows		3			3
Asset Retirement Obligations at end of period	$	48		$	43