Fair Value Measurments (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Debt carrying value	$ 3,378,000	$ 2,470,000
Estimated Fair Value	3,190,319	2,626,349
Notes Due 2015 [Member]
Debt Instrument [Line Items]
Debt carrying value	100,000	100,000
Estimated Fair Value	101,931	105,913
Debt Instruments Interst Rates	5.45%
Notes Due 2016 [Member]
Debt Instrument [Line Items]
Debt carrying value	62,000	62,000
Estimated Fair Value	65,027	70,228
Debt Instruments Interst Rates	7.31%
Notes Due 2017 [Member]
Debt Instrument [Line Items]
Debt carrying value	116,000	116,000
Estimated Fair Value	116,240	126,342
Debt Instruments Interst Rates	4.98%
Notes Due 2018 [Member]
Debt Instrument [Line Items]
Debt carrying value	200,000	200,000
Estimated Fair Value	203,738	226,127
Debt Instruments Interst Rates	5.92%
Notes Due December 2018 [Member]
Debt Instrument [Line Items]
Debt carrying value	450,000	450,000
Estimated Fair Value	414,505	466,946
Debt Instruments Interst Rates	5.75%
Notes Due 2019 [Member]
Debt Instrument [Line Items]
Debt carrying value	173,000	173,000
Estimated Fair Value	187,105	211,877
Debt Instruments Interst Rates	7.77%
Notes Due January 2020 [Member]
Debt Instrument [Line Items]
Debt carrying value	207,000	207,000
Estimated Fair Value	201,371	229,068
Debt Instruments Interst Rates	5.50%
Notes Due October 2020 [Member]
Debt Instrument [Line Items]
Debt carrying value	315,000	315,000
Estimated Fair Value	283,335	323,732
Debt Instruments Interst Rates	4.51%
Notes Due January 2022 [Member]
Debt Instrument [Line Items]
Debt carrying value	87,000	87,000
Estimated Fair Value	82,581	95,736
Debt Instruments Interst Rates	5.60%
Notes Due October 2022 [Member]
Debt Instrument [Line Items]
Debt carrying value	35,000	35,000
Estimated Fair Value	30,476	35,494
Debt Instruments Interst Rates	4.66%
Notes Due October 2024 [Member]
Debt Instrument [Line Items]
Debt carrying value	850,000	0
Estimated Fair Value	754,485	0
Debt Instruments Interst Rates	6.125%
Notes Due January 2025 [Member]
Debt Instrument [Line Items]
Debt carrying value	90,000	90,000
Estimated Fair Value	83,876	99,142
Debt Instruments Interst Rates	5.85%
Notes Due October 2025 [Member]
Debt Instrument [Line Items]
Debt carrying value	175,000	175,000
Estimated Fair Value	147,649	175,744
Debt Instruments Interst Rates	4.91%
Credit Facility [Member]
Debt Instrument [Line Items]
Debt carrying value	518,000	460,000
Estimated Fair Value	$ 518,000	$ 460,000