Revolving Line Of Credit (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Revolving credit facility, borrowing capacity	$ 150,000,000
Revolving credit facility, maximum borrowing capacity	200,000,000
Revolving credit facility maturity date	August 11, 2014
Effective average interest rate	3.30%	3.40%
Interest rate	3.40%
Outstanding borrowings	60,700,000
Unused available borrowing capacity	81,000,000
Letters of credit, outstanding	8,300,000
Interest paid	1,900,000	689,000
LIBOR Rate [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate margin over reference rate	3.75%
LIBOR Rate [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate margin over reference rate	2.50%
Bank Of America's Prime Rate [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest rate margin over reference rate	2.75%
Bank Of America's Prime Rate [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest rate margin over reference rate	1.50%
Letter Of Credit [Member]
Debt Instrument [Line Items]
Sublimit for letters of credit	$ 25,000,000