Derivative Instruments (Details) (Not Designated as Hedging Instrument [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Interest rate swap agreements [Abstract]
Notional Amount of Interest rate swap	$ 472,754	$ 515,278
Estimated Fair Value of Interest rate swap	(234)	(210)
Pay Fixed / Receive Variable Swaps [Member]
Interest rate swap agreements [Abstract]
Notional Amount of Interest rate swap	236,377	257,639
Estimated Fair Value of Interest rate swap	16,132	17,937
Pay Variable / Receive Fixed Swaps [Member]
Interest rate swap agreements [Abstract]
Notional Amount of Interest rate swap	236,377	257,639
Estimated Fair Value of Interest rate swap	$ (16,366)	$ (18,147)

Fair value as of the balance sheet date of interest rate derivative assets, net of interest rate derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized on the balance sheet.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 107

-Paragraph 10

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13495-108611

Name:	us-gaap_InterestRateDerivativesAtFairValueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant