Fair Value Measurments (Details 1) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Carrying Amount	$ 2,470,000,000	$ 1,837,000,000
Notes Due 2015 [Member]
Debt Instrument [Line Items]
Carrying Amount	100,000,000	100,000,000
Estimated Fair Value	105,913,000	107,801,000
Debt Instruments Interst Rates	5.45%
Notes Due 2016 [Member]
Debt Instrument [Line Items]
Carrying Amount	62,000,000	62,000,000
Estimated Fair Value	70,228,000	72,046,000
Debt Instruments Interst Rates	7.31%
Notes Due 2017 [Member]
Debt Instrument [Line Items]
Carrying Amount	116,000,000	116,000,000
Estimated Fair Value	126,342,000	127,109,000
Debt Instruments Interst Rates	4.98%
Notes Due 2018 [Member]
Debt Instrument [Line Items]
Carrying Amount	200,000,000	200,000,000
Estimated Fair Value	226,127,000	230,062,000
Debt Instruments Interst Rates	5.92%
Notes Due December 2018 [Member]
Debt Instrument [Line Items]
Carrying Amount	450,000,000	0
Estimated Fair Value	466,946,000	0
Debt Instruments Interst Rates	5.75%
Notes Due 2019 [Member]
Debt Instrument [Line Items]
Carrying Amount	173,000,000	173,000,000
Estimated Fair Value	211,877,000	219,045,000
Debt Instruments Interst Rates	7.77%
Notes Due January 2020 [Member]
Debt Instrument [Line Items]
Carrying Amount	207,000,000	207,000,000
Estimated Fair Value	229,068,000	234,552,000
Debt Instruments Interst Rates	5.50%
Notes Due October 2020 [Member]
Debt Instrument [Line Items]
Carrying Amount	315,000,000	315,000,000
Estimated Fair Value	323,732,000	331,329,000
Debt Instruments Interst Rates	4.51%
Notes Due January 2022 [Member]
Debt Instrument [Line Items]
Carrying Amount	87,000,000	87,000,000
Estimated Fair Value	95,736,000	98,526,000
Debt Instruments Interst Rates	5.60%
Notes Due October 2022 [Member]
Debt Instrument [Line Items]
Carrying Amount	35,000,000	35,000,000
Estimated Fair Value	35,494,000	36,361,000
Debt Instruments Interst Rates	4.66%
Notes Due January 2025 [Member]
Debt Instrument [Line Items]
Carrying Amount	90,000,000	90,000,000
Estimated Fair Value	99,142,000	102,096,000
Debt Instruments Interst Rates	5.85%
Notes Due October 2025 [Member]
Debt Instrument [Line Items]
Carrying Amount	175,000,000	175,000,000
Estimated Fair Value	175,744,000	179,677,000
Debt Instruments Interst Rates	4.91%
Credit Facility [Member]
Debt Instrument [Line Items]
Carrying Amount	460,000,000	277,000,000
Estimated Fair Value	$ 460,000,000	$ 277,000,000