Income taxes: (Tables)

12 Months Ended

Dec. 31, 2016

Income taxes:

Summary of difference between statutory federal income tax rate on income before income taxes and effective income tax rate

	2016		2015		2014
Statutory federal income tax rate	35.0	%	35.0	%	35.0%
Patronage exclusion	(34.7	%)	(34.7	%)	(34.4%)
Other	(0.3	%)	(0.3	%)	(0.6%)
Effective income tax rate	0.0	%	0.0	%	0.0%

Schedule of components of net deferred tax assets and liabilities

		(dollars in thousands)
		2016			2015
Deferred tax assets
Net operating losses	$	29,724		$	29,724
Tax credits (alternative minimum tax and other)		599			1,198
Accounting for Rocky Mountain transactions		349,127			348,779
Other assets		109,793			107,527
Deferred tax assets		489,243			487,228
Less: Valuation allowance		(29,724	)		(29,724	)
Net deferred tax assets	$	459,519		$	457,504
Deferred tax liabilities
Depreciation	$	435,570		$	426,173
Accounting for Rocky Mountain transactions		170,402			167,907
Other liabilities		123,121			131,454
Deferred tax liabilities		729,093			725,534
Net deferred tax liabilities		269,574			268,030
Less: Patronage exclusion		(269,574	)		(268,030	)
Net deferred taxes	$	–		$	–

Schedule of federal tax net operating loss carryforwards and alternative minimum tax credits

    As of December 31, 2016, we have federal tax net operating loss carryforwards and alternative minimum tax credits as follows:

		(dollars in thousands)
Expiration Date		Alternative Minimum Tax Credits		NOLs
2018	$	–	$	61,533
2019		–		10,516
2020	$	–		4,362
None	$	599	$	–
	$	599	$	76,411