Non-Qualified Retirement, Savings and Investment Plans (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2009	Dec. 31, 2011
Schedule of Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Number of non-qualified retirement savings and investment plans	2		2
Investments, employee benefit plans, at fair value	$ 12,530,000		$ 12,530,000			$ 11,678,000
Executive Deferred Compensation Plan [Member]
Schedule of Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Basis points					300
Deferred Compensation Liability, Current and Noncurrent	14,800,000		14,800,000			17,200,000
Assets Held-in-trust	6,100,000		6,100,000			14,200,000
Assets Held-in-trust, Current	3,700,000		3,700,000
Investment gains (losses)	100,000	(1,400,000)	1,200,000	(900,000)
Common stock held by non-qualified plan	100,000		100,000
General and Administrative Expense [Member]
Schedule of Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Compensation expense	200,000	17,000	700,000	500,000
Non-Qualified Plan [Member]
Schedule of Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Deferred Compensation Liability, Current and Noncurrent	11,100,000		11,100,000			10,400,000
Investment gains (losses)	400,000	(1,200,000)	1,000,000	(900,000)
Common stock held by non-qualified plan	1,000,000		1,000,000			900,000
Increase (decrease) in compensation expense	200,000	(1,300,000)	800,000	(1,100,000)
Investments, employee benefit plans, at fair value	$ 10,100,000		$ 10,100,000			$ 9,500,000