Long-term Debt - Narrative (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Debt Instrument [Line Items]
Revolving credit facility, average borrowings	$ 34,300,000	$ 24,800,000
Minimum amount borrowed at any one time outstanding	15,000,000	0
Maximum amount borrowed at any one time outstanding	73,300,000	72,900,000
Revolving credit facility, weighted-average interest rate	2.00%	2.10%
Total unused facility fees and letter of credit participation fees	200,000	200,000
Unamortized debt issuance costs	1,400,000		1,500,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility	200,000,000
Revolving credit facility, potentially additional borrowing capacity	$ 100,000,000
Debt instrument, description of variable rate basis	LIBOR or CDOR
Revolving Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Revolving credit facility, margin added to variable interest rate	1.75%
Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Revolving credit facility, margin added to variable interest rate	2.25%