Derivative Instruments (Details Textual) (USD $) In Thousands, unless otherwise specified	1 Months Ended	12 Months Ended				36 Months Ended			6 Months Ended
	Aug. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2007 Agreement	Dec. 15, 2010	Dec. 31, 2012	Jun. 30, 2011	Jun. 30, 2011 Interest Rate Swap Agreement 1 [Member]	Jun. 30, 2011 Interest Rate Swap Agreement 2 [Member]
Derivative Instruments (Textual)
Variable rate basis					3-month LIBOR	6-month LIBOR			6-month LIBOR	6-month LIBOR
Interest rate spread above London Interbank Offered Rate (LIBOR)					1.25%	1.25%			4.05%	8.03%
Fixed interest rate under swap agreement					5.25%				6.50%	10.125%
Notional amount of interest rate swap agreement					$ 325,000				$ 500,000	$ 150,000
Notional amount for forward starting interest rate swap agreements					1,500,000
Aggregate notional amount of swaps					325,000			500,000
Estimated amount of pretax loss in AOCI related to interest rate swap that would be reclassified to earnings							6,395
Amount of pretax loss accumulated in Other Comprehensive Income related to interest rate swap reclassified to earnings			12,075
Proceeds from (payments for) settlement of interest rate swap agreements		23,387	0	0	(89,777,000)
Number of forward starting interest rate swap agreements					15
Length of interest rate swap agreement (In years)					3 years
Cash proceeds from interest rate swap agreements	25,382
Accrued interest income	1,995
Forward component of the settlement	23,387
Interest expense amortized		$ 1,291