Fair Value (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 item	Mar. 31, 2013 Carrying Value	Dec. 31, 2012 Carrying Value	Mar. 31, 2013 Fair Value	Dec. 31, 2012 Fair Value
Grouping Financial Statement Captions
Long-term debt		$ 5,950,994	$ 5,930,449	$ 7,106,408	$ 7,213,365
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%