Fair Value Measurments (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Carrying Amount	$ 1,837,000	$ 1,903,000
Notes Due 2015 [Member]
Debt Instrument [Line Items]
Carrying Amount	100,000	100,000
Estimated Fair Value	107,801	111,475
Private Debt Placement Instruments Interst Rates	5.45%
Notes Due 2016 [Member]
Debt Instrument [Line Items]
Carrying Amount	62,000	62,000
Estimated Fair Value	72,046	74,817
Private Debt Placement Instruments Interst Rates	7.31%
Notes Due 2017 [Member]
Debt Instrument [Line Items]
Carrying Amount	116,000	116,000
Estimated Fair Value	127,109	128,570
Private Debt Placement Instruments Interst Rates	4.98%
Notes Due 2018 [Member]
Debt Instrument [Line Items]
Carrying Amount	200,000	200,000
Estimated Fair Value	230,062	231,091
Private Debt Placement Instruments Interst Rates	5.92%
Notes Due 2019 [Member]
Debt Instrument [Line Items]
Carrying Amount	173,000	173,000
Estimated Fair Value	219,045	219,552
Private Debt Placement Instruments Interst Rates	7.77%
Notes Due January 2020 [Member]
Debt Instrument [Line Items]
Carrying Amount	207,000	207,000
Estimated Fair Value	234,552	229,423
Private Debt Placement Instruments Interst Rates	5.50%
Notes Due October 2020 [Member]
Debt Instrument [Line Items]
Carrying Amount	315,000	315,000
Estimated Fair Value	331,329	318,925
Private Debt Placement Instruments Interst Rates	4.51%
Notes Due January 2022 [Member]
Debt Instrument [Line Items]
Carrying Amount	87,000	87,000
Estimated Fair Value	98,526	94,165
Private Debt Placement Instruments Interst Rates	5.60%
Notes Due October 2022 [Member]
Debt Instrument [Line Items]
Carrying Amount	35,000	35,000
Estimated Fair Value	36,361	34,631
Private Debt Placement Instruments Interst Rates	4.66%
Notes Due January 2025 [Member]
Debt Instrument [Line Items]
Carrying Amount	90,000	90,000
Estimated Fair Value	102,096	99,022
Private Debt Placement Instruments Interst Rates	5.85%
Notes Due October 2025 [Member]
Debt Instrument [Line Items]
Carrying Amount	175,000	175,000
Estimated Fair Value	179,677	173,835
Private Debt Placement Instruments Interst Rates	4.91%
Credit Facility [Member]
Debt Instrument [Line Items]
Carrying Amount	277,000	343,000
Estimated Fair Value	$ 277,000	$ 343,000