Fair Value (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Fair Value
Fair value of Level 3 assets	$ 51,600,000		$ 51,600,000
Percentage of level 3 assets to total invested assets	0.50%		0.50%
Long-term debt, carrying value	1,664,015,000		1,664,015,000		1,668,849,000
Long-term debt, fair value	1,824,300,000		1,824,300,000		1,746,500,000
Material transfers between Level 1 and Level 2	$ 0	$ 0	$ 0	$ 0