Fair Value: Change in Level 3 assets (Details) - Interest rate options - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Assets (Liabilities):
Balance at the beginning of the period	$ 4,371	$ 63,471
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)	(3,361)	(59,100)
Balance at the end of the period	$ 1,010	$ 4,371