Income taxes: (Tables)

12 Months Ended

Dec. 31, 2018

Income taxes:

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

	2018		2017		2016
Statutory federal income tax rate	21.0	%	35.0	%	35.0	%
Patronage exclusion	(20.8)	%	(34.1)	%	(34.7)	%
AMT credit monetization	(0.0)	%	2.2	%	0.0	%
Other	(0.2)	%	(0.9)	%	(0.3)	%
Effective income tax rate	(0.0)	%	(2.2)	%	0.0	%

Schedule of components of net deferred tax assets and liabilities

	(dollars in thousands)
	2018		2017
Deferred tax assets
Net operating losses	$	3,830	$	19,668
Tax credits (alternative minimum tax and other)		—		—
Accounting for Rocky Mountain transactions		231,543		231,268
Advance payments		46,708		—
Other assets		82,655		75,013
Deferred tax assets		364,736		325,949
Less: Valuation allowance		(3,830)		(19,668)
Net deferred tax assets	$	360,906	$	306,281
Deferred tax liabilities
Depreciation	$	268,039	$	271,652
Accounting for Rocky Mountain transactions		116,226		114,514
Other liabilities		75,691		78,407
Deferred tax liabilities		459,956		464,573
Net deferred tax liabilities		99,050		158,292
Less: Patronage exclusion		(99,050)		(158,292)
Net deferred taxes	$	—	$	—

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

As of December 31, 2018, we have federal tax net operating loss carryforwards and alternative minimum tax credits as follows:

	(dollars in thousands)
	Alternative
	Minimum
Expiration Date	Tax Credits		NOLs
2019		—		10,516
2020		—		4,362
	$	—	$	14,878