Interest Rate Swap Agreements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 Agreement	Mar. 31, 2011
Interest rate swap agreements designated as hedge agreements
Amount Designated as a Hedge	$ 620,615
Nominal amount	650,000
Current Liability	8,701
Long-Term Liability	6,522
Estimated Total Fair Value at March 31, 2012	15,223
Interest Rate Swap Agreements (Textual) [Abstract]
Nominal amount of interest rate swap	650,000
Interest Rate Swap Agreements (Additional Textual) [Abstract]
Number of interest rate swap agreements	5
Changes in fair values are reclassified from accumulated other comprehensive income (loss) into earnings	3,107	3,992
Amortized Interest Expenses related to a previously terminated interest rate swap agreement	988	1,158
Interest expense on Interest rate swap agreement will amortize over the next twelve months	1,482
Interest Rate Swap One Agreement [Member]
Interest rate swap agreements designated as hedge agreements
Amount Designated as a Hedge	106,632
Nominal amount	125,000
Effective Date	Aug. 01, 2007
Pay Rate	4.922%
Receive Rate	3-Month LIBOR
Expiration Date	Aug. 12, 2012
Current Liability	2,079
Estimated Total Fair Value at March 31, 2012	2,079
Interest Rate Swap Agreements (Textual) [Abstract]
Nominal amount of interest rate swap	125,000
Interest Rate Swap One Agreement [Member] | Nondesignated [Member]
Interest Rate Swap Agreements (Textual) [Abstract]
Portion of interest rate swap not designated as hedge	18,368
Interest Rate Swap Two Agreement [Member]
Interest rate swap agreements designated as hedge agreements
Amount Designated as a Hedge	63,983
Nominal amount	75,000
Effective Date	Nov. 01, 2008
Pay Rate	3.63%
Receive Rate	1-Month LIBOR
Expiration Date	Nov. 01, 2012
Current Liability	1,591
Estimated Total Fair Value at March 31, 2012	1,591
Interest Rate Swap Agreements (Textual) [Abstract]
Nominal amount of interest rate swap	75,000
Interest Rate Swap Two Agreement [Member] | Nondesignated [Member]
Interest Rate Swap Agreements (Textual) [Abstract]
Portion of interest rate swap not designated as hedge	11,017
Interest Rate Swap Three Agreement [Member]
Interest rate swap agreements designated as hedge agreements
Nominal amount	175,000
Effective Date	Dec. 01, 2010
Pay Rate	1.3975%
Receive Rate	1-Month LIBOR
Expiration Date	Sep. 01, 2015
Current Liability	1,827
Long-Term Liability	2,231
Estimated Total Fair Value at March 31, 2012	4,058
Interest Rate Swap Agreements (Textual) [Abstract]
Nominal amount of interest rate swap	175,000
Interest Rate Swap Four Agreement [Member]
Interest rate swap agreements designated as hedge agreements
Nominal amount	175,000
Effective Date	Dec. 01, 2010
Pay Rate	1.40%
Receive Rate	1-Month LIBOR
Expiration Date	Sep. 01, 2015
Current Liability	1,842
Long-Term Liability	2,292
Estimated Total Fair Value at March 31, 2012	4,134
Interest Rate Swap Agreements (Textual) [Abstract]
Nominal amount of interest rate swap	175,000
Interest Rate Swap Five Agreement [Member]
Interest rate swap agreements designated as hedge agreements
Nominal amount	100,000
Effective Date	Nov. 01, 2011
Pay Rate	1.715%
Receive Rate	1-Month LIBOR
Expiration Date	Apr. 01, 2016
Current Liability	1,362
Long-Term Liability	1,999
Estimated Total Fair Value at March 31, 2012	3,361
Interest Rate Swap Agreements (Textual) [Abstract]
Nominal amount of interest rate swap	$ 100,000