Summary of significant accounting policies: - Asset retirement obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Asset Retirement Obligations
Balance at the beginning of the period	$ 602,230	$ 432,260
Liabilities settled	(1,260)	(299)
Accretion	32,361	26,108	$ 24,616
Change in Cash Flow Estimates	64,720	144,161
Balance at the end of the period	698,051	602,230	432,260
Nuclear
Asset Retirement Obligations
Balance at the beginning of the period	488,458	369,046
Accretion	29,107	23,231
Change in Cash Flow Estimates		96,181
Balance at the end of the period	517,565	488,458	369,046
Coal Ash Pond
Asset Retirement Obligations
Balance at the beginning of the period	93,622	42,609
Liabilities settled	(553)
Accretion	2,215	1,929
Change in Cash Flow Estimates	61,181	49,084
Balance at the end of the period	156,465	93,622	42,609
Other
Asset Retirement Obligations
Balance at the beginning of the period	20,150	20,605
Liabilities settled	(707)	(299)
Accretion	1,039	948
Change in Cash Flow Estimates	3,539	(1,104)
Balance at the end of the period	$ 24,021	$ 20,150	$ 20,605