Indebtedness (Details) (USD $)	12 Months Ended			1 Months Ended	12 Months Ended							1 Months Ended	3 Months Ended		1 Months Ended
	Dec. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Jan. 31, 2006 Senior Notes agreement	Dec. 31, 2012 Senior Notes quarter tranche	Dec. 31, 2012 Senior Notes Maximum	Dec. 31, 2012 Senior Notes Minimum	Dec. 31, 2012 Senior 5.0% unsecured notes due, 2018 ("Series A Notes")	Dec. 31, 2011 Senior 5.0% unsecured notes due, 2018 ("Series A Notes")	Dec. 31, 2012 Senior 5.75% unsecured notes due, 2021 ("Series B 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	Jan. 13, 2006 Senior 5.60% unsecured notes due, 2011	Aug. 31, 2010 Credit Facility	Dec. 31, 2012 Credit Facility
Indebtedness
Fair value of outstanding indebtedness		$ 208,800,000
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				5 years
Repurchase of debt	(10,000,000)												10,000,000
Consolidated leverage ratio						3.0
Consolidated interest coverage ratio							4.0
Number of consecutive quarters for which the maximum consolidated leverage ratio is required to be maintained under financial covenants					4
Number of consecutive quarters for which the minimum consolidated interest coverage ratio is required to be maintained under financial covenants					1 year
Number of tranches of debt issuance					2
Revolving credit facility term															3 years
Maximum borrowing capacity																125,000,000
Amount that borrowing capacity can be expanded upon entity's request																200,000,000
Summary of long-term debt
Total long-term debt		$ 190,000,000	$ 190,000,000					$ 95,000,000	$ 95,000,000	$ 95,000,000	$ 95,000,000