Fair Value: (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2012 Carrying Value	Dec. 31, 2011 Carrying Value	Dec. 31, 2012 Fair Value	Dec. 31, 2011 Fair Value
Grouping Financial Statement Captions
Long-term debt		$ 5,930,449	$ 5,692,504	$ 7,213,365	$ 6,908,763
Number of primary sources of long-term debt	3
Description of Variable Rate Basis	LIBOR
Basis Spread on Variable Rate (as a percent)	1.25%