Fair Value Measurments (Details 1) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Debt carrying value	$ 3,390,000	$ 3,378,000
Estimated Fair Value	2,220,100	3,190,319
Notes Due 2015 [Member]
Debt Instrument [Line Items]
Debt carrying value	0	100,000
Estimated Fair Value	$ 0	101,931
Debt Instruments Interst Rates	5.45%
Notes Due 2016 [Member]
Debt Instrument [Line Items]
Debt carrying value	$ 62,000	62,000
Estimated Fair Value	$ 63,604	65,027
Debt Instruments Interst Rates	7.31%
Notes Due 2017 [Member]
Debt Instrument [Line Items]
Debt carrying value	$ 116,000	116,000
Estimated Fair Value	$ 113,420	116,240
Debt Instruments Interst Rates	4.98%
Notes Due 2018 [Member]
Debt Instrument [Line Items]
Debt carrying value	$ 200,000	200,000
Estimated Fair Value	$ 191,985	203,738
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	$ 111,451	414,505
Debt Instruments Interst Rates	5.75%
Notes Due 2019 [Member]
Debt Instrument [Line Items]
Debt carrying value	$ 173,000	173,000
Estimated Fair Value	$ 174,488	187,105
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	$ 185,052	201,371
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	$ 258,520	283,335
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	$ 73,034	82,581
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	$ 25,558	30,476
Debt Instruments Interst Rates	4.66%
Notes Due October 2024 [Member]
Debt Instrument [Line Items]
Debt carrying value	$ 850,000	850,000
Estimated Fair Value	$ 206,321	754,485
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	$ 70,756	83,876
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	$ 115,911	147,649
Debt Instruments Interst Rates	4.91%
Credit Facility [Member]
Debt Instrument [Line Items]
Debt carrying value	$ 630,000	518,000
Estimated Fair Value	$ 630,000	$ 518,000