Fair Value (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Fair Value
Fair value of Level 3 assets	$ 52,300,000		$ 52,300,000
Percentage of level 3 assets to total invested assets, less than	0.50%		0.50%
Long-term debt, carrying value	1,661,552,000		1,661,552,000		1,668,849,000
Long-term debt, fair value	1,837,500,000		1,837,500,000		1,746,500,000
Material transfers between Level 1 and Level 2	$ 0	$ 0	$ 0	$ 0