Debt Narrative (Details) (USD $)	12 Months Ended					0 Months Ended	12 Months Ended			12 Months Ended								12 Months Ended
	Dec. 31, 2011 subsidiaries	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Line of Credit [Member] Base Rate [Member] Minimum	Dec. 31, 2011 Line of Credit [Member] Base Rate [Member] Maximum	Feb. 24, 2011 Revolver [Member] Line of Credit [Member]	Dec. 31, 2011 Revolver [Member] Line of Credit [Member]	Dec. 31, 2010 Revolver [Member] Line of Credit [Member]	Jun. 16, 2006 Revolver [Member] Line of Credit [Member]	Dec. 31, 2011 Revolver [Member] Line of Credit [Member] Minimum	Dec. 31, 2011 Revolver [Member] Line of Credit [Member] Maximum	Dec. 31, 2011 Revolver [Member] Line of Credit [Member] LIBOR Rate [Member] Minimum	Dec. 31, 2011 Revolver [Member] Line of Credit [Member] LIBOR Rate [Member] Maximum	Feb. 24, 2011 Swingline Loans [Member] Line of Credit [Member]	Feb. 24, 2011 Letter of Credit [Member] Line of Credit [Member]	Dec. 31, 2011 Senior Unsecured Revolving Credit Facility Maturing 12 31 2010 [Member] Line of Credit [Member]	Jun. 16, 2006 Senior Unsecured Revolving Credit Facility Maturing 12 31 2010 [Member] Line of Credit [Member]	Dec. 31, 2011 Senior Notes [Member] Unsecured Debt [Member]	Dec. 31, 2010 Senior Notes [Member] Unsecured Debt [Member]	Feb. 24, 2011 Senior Notes [Member] Unsecured Debt [Member]	Aug. 25, 2010 Senior Notes [Member] Unsecured Debt [Member]
Debt instrument face amount						$ 300,000,000			$ 350,000,000								$ 350,000,000				$ 250,000,000
Termination of Unsecured Revolving Credit Agreement						350,000,000
Line of credit maximum borrowing capacity														20,000,000	30,000,000
Debt instrument maturity date						Feb. 24, 2016												Aug. 28, 2020
Senior notes coupon rate																				5.70%
Debt instrument additional borrowing capacity						150,000,000
Debt Instrument, Basis Spread on Variable Rate				0.05%	0.80%							1.05%	1.80%
Line of Credit Facility, Commitment Fee Percentage										0.20%	0.45%
Total Leverage Ratio											3.5
Interest Coverage Ratio											3.5
Payments of Debt Issuance Costs	2,356,000	800,000	0				2,400,000
Debt Instrument, Unamortized Discount																					600,000
Debt instruments effective interest rate								0.675%													6.19%
Senior notes to be paid																		semi-annual	semi-annually
Number of subsidiaries owned	8
Percentage of ownership in domestic subsidiaries	50.00%																	100.00%
Unsecured revolving credit facility outstanding																$ 0
Debt Instrument Percentage Of Principal Amount To Be Redeemed By Company																		100.00%
Debt Instrument Basis Spread On Variable Rate Redemption Option																		0.45%