Debt (Senior Secured Credit Facility With PNC Bank, N.A.) (Narrative) (Details) (USD $)			3 Months Ended			1 Months Ended	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 PNC Bank, N.A. [Member]	Mar. 31, 2013 Credit Facility LIBOR Plus Applicable Spread [Member] PNC Bank, N.A. [Member]	Mar. 31, 2013 Credit Facility PRIME Plus Applicable Spread [Member] PNC Bank, N.A. [Member]	Jul. 31, 2011 Amended And Restated [Member] PNC Bank, N.A. [Member]	Mar. 31, 2013 Amended And Restated [Member] PNC Bank, N.A. [Member]	Mar. 31, 2013 Letters of Credit [Member] PNC Bank, N.A. [Member]	Jul. 31, 2011 Letters of Credit [Member] PNC Bank, N.A. [Member]
Line of Credit Facility [Line Items]
Credit facility, maximum available						$ 300,000,000			$ 75,000,000
Line of credit facility option to increase maximum borrowing capacity						50,000,000
Debt issuance costs						2,800,000
Line of credit facility, expiration date							Jul. 01, 2016
Line of credit facility, amount outstanding	161,480,000	157,250,000		155,000,000	6,500,000
Incremental interest rate above variable rate				2.75%	1.75%
Effective interest rate	5.00%			2.95%	5.00%
Outstanding letters of credit								22,100,000
Long-term debt, percentage bearing fixed interest, percentage								2.75%
Maximum borrowing capacity measured as a multiple of EBITDA			300.00%
EBITDA measurement period for determining maximum borrowing capacity			12 months
Line of credit facility, remaining borrowing capacity			$ 51,300,000