Non-Qualified Retirement, Savings and Investment Plans (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2009	Dec. 31, 2012
Schedule of Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Number of non-qualified retirement savings and investment plans	2
Investments, employee benefit plans, at fair value	$ 13,933,000			$ 12,755,000
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 long-term	10,100,000			11,700,000
Common stock held by EDCP plan	200,000			100,000
Assets held-in-trust	3,400,000			6,000,000
Assets held-in-trust, current	400,000
Investment gains	100,000	1,100,000
General and Administrative Expense [Member]
Schedule of Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Compensation expense	300,000	400,000
Non-Qualified Plan [Member]
Schedule of Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Deferred compensation liability, current and long-term	12,200,000			11,200,000
Common stock held by EDCP plan	1,200,000			1,000,000
Investment gains	600,000	900,000
Increase in compensation expense	800,000	900,000
Investments, employee benefit plans, at fair value	$ 10,900,000			$ 10,200,000