Fair-Value Measurements (Narrative) (Details) (USD $)	3 Months Ended					3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Credit Agreement [Member]	Dec. 31, 2010 Credit Agreement [Member]	Mar. 31, 2013 Interest Rate Swap [Member]	Mar. 31, 2012 Interest Rate Swap [Member]	Mar. 31, 2013 Interest Rate Swap [Member] Designated as Hedging Instrument [Member] Other Noncurrent Liabilities [Member]	Dec. 31, 2012 Interest Rate Swap [Member] Designated as Hedging Instrument [Member] Other Noncurrent Liabilities [Member]	Mar. 31, 2013 Interest Rate Swap [Member] Not Designated as Hedging Instrument [Member]	Mar. 31, 2013 Interest Rate Swap [Member] Minimum [Member]	Mar. 31, 2013 Interest Rate Swap [Member] Maximum [Member]	Jul. 16, 2010 Interest Rate Swap [Member] Credit Agreement [Member]
Fair Value Disclosures [Line Items]
Amount of credit facility				$ 330,000,000	$ 390,000,000
Notional amount													105,000,000
Fixed interest rate in swap													2.04%
Credit spread											2.00%	2.75%
Liability Derivatives	4,261,000		4,718,000					4,261,000	4,718,000	4,300,000
Liability for fixed rate leg										5,400,000
Receivable for floating rate leg										1,100,000
Interest expense	$ 4,324,000	$ 4,728,000				$ 500,000	$ 400,000