Indebtedness (Details) (USD $)	9 Months Ended						1 Months Ended	3 Months Ended
	Sep. 30, 2011 Quarter D	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 Senior Notes Tranch	Sep. 30, 2011 Senior 5.0% unsecured notes due, 2018 ("Series A Notes")	Sep. 30, 2011 Senior 5.75% unsecured notes due, 2021 ("Series B Notes")	Jan. 31, 2006 Senior 5.60% unsecured notes due, 2011	Mar. 31, 2010 Senior 5.60% unsecured notes due, 2011	Dec. 31, 2010 Senior 5.60% unsecured notes due, 2011	Jan. 13, 2006 Senior 5.60% unsecured notes due, 2011
Indebtedness
Fair value of outstanding indebtedness	$ 188,200,000
Indebtedness
Face amount of Long-term debt							200,000,000
Interest rate of debt (as a percent)					5.00%	5.75%				5.60%
Proceeds from issuance of debt							198,200,000
Repurchase of debt		(10,000,000)						10,000,000
Numerator for the consolidated leverage ratio, maximum	3.0
Denominator for the consolidated leverage ratio, maximum	1.0
Number of consecutive quarters for which the maximum consolidated leverage ratio is required to be maintained under financial covenants	4
Numerator for the consolidated interest coverage ratio, minimum	4.0
Denominator for the consolidated interest coverage ratio, minimum	1.0
Number of consecutive quarters for which the minimum consolidated interest coverage ratio is required to be maintained under financial covenants	4
Number of tranches of debt issuance				2
Summary of long-term debt
Discount on long-term debt									(1,000)
Total long-term debt	$ 190,000,000		$ 189,999,000		$ 95,000,000	$ 95,000,000			$ 190,000,000