Derivative Financial Instruments (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
Jun. 30, 2014 agreement	Jun. 30, 2013
Derivative Financial Instruments [Abstract]
Number of interest rate swap agreements	7
Number of fuel hedge agreements	1
Ineffectiveness recognized on the fuel hedge	$ 0	$ 0

Number of interest rate derivative instruments held by the entity at the reporting date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1B

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1A

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5579245-113959

Name:	us-gaap_NumberOfInterestRateDerivativesHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant