Income taxes: (Tables)

12 Months Ended

Dec. 31, 2015

Income taxes:

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

	2015		2014		2013
Statutory federal income tax rate	35.0	%	35.0	%	35.0%
Patronage exclusion	(34.7	%)	(34.4	%)	(33.3%)
Other	(0.3	%)	(0.6	%)	(1.7%)
Effective income tax rate	0.0	%	0.0	%	0.0%

Schedule of components of net deferred tax assets and liabilities

		(dollars in thousands)
		2015			2014
Deferred tax assets
Net operating losses	$	29,724		$	29,724
Tax credits (alternative minimum tax and other)		1,198			1,275
Accounting for Rocky Mountain transactions		348,779			348,460
Other assets		107,527			100,203
Deferred tax assets		487,228			479,662
Less: Valuation allowance		(29,724	)		(30,999	)
Net deferred tax assets	$	457,504		$	448,663
Deferred tax liabilities
Depreciation	$	426,173		$	444,139
Accounting for Rocky Mountain transactions		167,907			165,462
Other liabilities		131,454			112,732
Deferred tax liabilities		725,534			722,333
Net deferred tax liabilities		268,030			273,670
Less: Patronage exclusion		(268,030	)		(273,670	)
Net deferred taxes	$	–		$	–

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

    As of December 31, 2015, 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		1,198		–
	$	1,198	$	76,411