Interest Rate Swap Agreements (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Interest rate swap agreements designated as hedge agreements
Amount Designated as a Hedge	$ 620,615
Nominal amount	650,000
Current Liability	6,783
Long-Term Liability	9,206
Estimated Total Fair Value at Jun 30, 2012	15,989
Interest Rate Swap One Agreements [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	680
Long-Term Liability
Estimated Total Fair Value at Jun 30, 2012	680
Interest Rate Swap Two Agreements [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	960
Long-Term Liability
Estimated Total Fair Value at Jun 30, 2012	960
Interest Rate Swap Three Agreements [Member]
Interest rate swap agreements designated as hedge agreements
Amount Designated as a Hedge	175,000
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	1,823
Long-Term Liability	3,198
Estimated Total Fair Value at Jun 30, 2012	5,021
Interest Rate Swap Four Agreements [Member]
Interest rate swap agreements designated as hedge agreements
Amount Designated as a Hedge	175,000
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	1,845
Long-Term Liability	3,233
Estimated Total Fair Value at Jun 30, 2012	5,078
Interest Rate Swap Five Agreements [Member]
Interest rate swap agreements designated as hedge agreements
Amount Designated as a Hedge	100,000
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	1,475
Long-Term Liability	2,775
Estimated Total Fair Value at Jun 30, 2012	$ 4,250