Indebtedness (Details) (USD $)	12 Months Ended		1 Months Ended	12 Months Ended					1 Months Ended	3 Months Ended			1 Months Ended		1 Months Ended
	Dec. 31, 2010	Dec. 31, 2011	Jan. 31, 2006 Senior Notes Y agreement	Dec. 31, 2011 Senior Notes Q tranche	Dec. 31, 2011 Senior Notes Maximum	Dec. 31, 2011 Senior Notes Minimum	Dec. 31, 2011 Senior 5.0% unsecured notes due, 2018 ("Series A Notes")	Dec. 31, 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	Oct. 31, 2009 Credit Facility D	Oct. 05, 2009 Credit Facility	Aug. 31, 2010 New Credit Facility Y	Dec. 31, 2011 New Credit Facility
Indebtedness
Fair value of outstanding indebtedness		$ 191,600,000
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
Number of forward interest rate swap agreements terminated			2
Net cash settlement received			1,100,000
Period over which gain was amortized as a reduction to interest expense (in years)			5
Repurchase of debt	(10,000,000)									10,000,000
Numerator for the consolidated leverage ratio					3.0
Denominator for the consolidated leverage ratio					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						4.0
Denominator for the consolidated interest coverage ratio						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
Revolving credit facility term (in days)													364		3
Maximum borrowing capacity														125,000,000		125,000,000
Amount that borrowing capacity can be expanded upon entity's request																200,000,000
Summary of long-term debt
Discount on long-term debt											(1,000)
Total long-term debt	$ 189,999,000	$ 190,000,000					$ 95,000,000	$ 95,000,000			$ 190,000,000