Debt: (Details)	Mar. 23, 2015 USD ($) item	Jan. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Feb. 20, 2014 USD ($) item
Maturities for long-term debt and capital lease obligations
Weighted average interest rate on short-term borrowings (as a percent)			0.43%	0.28%
Principal			$ 7,575,027,000	$ 7,256,995,000
Unamortized Debt Issuance Costs and Debt Discounts			97,988,000	$ 101,939,000
Long Term Debt And Capital Lease Obligations
Maturities for long-term debt and capital lease obligations
2016			189,840,000
2017			187,698,000
2018			193,457,000
2019			511,459,000
2020			345,556,000
Long Term Debt
Maturities for long-term debt and capital lease obligations
2016			185,885,000
2017			183,294,000
2018			188,552,000
2019			505,997,000
2020			$ 339,474,000
Weighted average interest rate on long-term debt			4.45%	4.55%
Facility
Debt
Borrowings guaranteed			$ 1,180,628,000
Maturities for long-term debt and capital lease obligations
2016			145,846,000
2017			143,110,000
2018			148,210,000
2019			153,233,000
2020			156,998,000
Principal			3,777,540,000	$ 3,456,953,000
Unamortized Debt Issuance Costs and Debt Discounts			56,851,000	58,417,000
Line of credit, amount outstanding			270,000,000
Number of future advance promissory notes | item					2
Facility | Maximum
Debt
Capitalized interest					$ 335,471,604
Aggregate borrowings					$ 3,057,069,461
FMBs
Maturities for long-term debt and capital lease obligations
2016			1,010,000
2017			1,010,000
2018			1,010,000
2019			351,010,000
2020			1,010,000
Principal			2,809,093,000	2,810,103,000
Unamortized Debt Issuance Costs and Debt Discounts			32,002,000	33,782,000
PCBs
Maturities for long-term debt and capital lease obligations
2016			37,352,000
2017			37,352,000
2018			37,352,000
2020			181,075,000
Principal			980,770,000	980,770,000
Unamortized Debt Issuance Costs and Debt Discounts			9,135,000	9,740,000
Series 2009 and Series 2010 pollution control bonds
Maturities for long-term debt and capital lease obligations
2016			37,000,000
2017			37,000,000
2018			37,000,000
2020			134,000,000
CFC
Maturities for long-term debt and capital lease obligations
2016			891,000
2017			937,000
2018			984,000
2019			1,035,000
2020			391,000
Principal			4,238,000	5,085,000
CoBank
Maturities for long-term debt and capital lease obligations
2016			786,000
2017			885,000
2018			996,000
2019			719,000
Principal			3,386,000	$ 4,084,000
Capital Lease Obligations.
Maturities for long-term debt and capital lease obligations
2016			3,955,000
2017			4,404,000
2018			4,905,000
2019			5,462,000
2020			6,082,000
Committed credit arrangements
Debt
Aggregate borrowings	$ 1,210,000,000		$ 1,610,000,000
Maturities for long-term debt and capital lease obligations
Number of separate facilities | item			4
Term of credit facility	5 years
Number of lenders | item	13
Letter of credit
Debt
Aggregate borrowings			$ 760,000,000
Maturities for long-term debt and capital lease obligations
Available borrowing capacity			509,000,000
Letters of credit, amount outstanding			251,000,000
Commercial paper
Maturities for long-term debt and capital lease obligations
Line of credit, amount outstanding			261,000,000
Rural Utilities Service Guaranteed Loans
Debt
Principal amount		$ 7,998,000	$ 153,637,000