Fair Value Measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair Value of Financial Instruments
Carrying Value	$ 3,886,960	$ 3,983,490
Fair Value	4,204,394	4,251,473
Subsidiary preferred stock, carrying value	69,188	69,188
Subsidiary preferred stock, fair value	69,000	69,000
2.35% senior notes due September 2016
Fair Value of Financial Instruments
Carrying Value	349,836	349,820
Fair Value	357,851	354,694
Interest rate on senior notes due (as a percent)	2.35%
6.15% senior notes due February 2018
Fair Value of Financial Instruments
Carrying Value	970,234	969,928
Fair Value	1,102,511	1,097,480
Interest rate on senior notes due (as a percent)	6.15%
9.25% senior notes due January 2019
Fair Value of Financial Instruments
Carrying Value	339,607	339,607
Fair Value	427,966	428,733
Interest rate on senior notes due (as a percent)	9.25%
5.00% senior notes due September 2020
Fair Value of Financial Instruments
Carrying Value	698,208	697,947
Fair Value	748,720	731,955
Interest rate on senior notes due (as a percent)	5.00%
4.625% senior notes due September 2021
Fair Value of Financial Instruments
Carrying Value	698,024	698,148
Fair Value	723,030	709,793
Interest rate on senior notes due (as a percent)	4.625%
5.10% senior notes due September 2023
Fair Value of Financial Instruments
Carrying Value	348,797	348,765
Fair Value	362,250	349,731
Interest rate on senior notes due (as a percent)	5.10%
Revolving Credit Facility
Fair Value of Financial Instruments
Carrying Value	117,500	170,000
Fair Value	117,500	170,000
Commercial paper
Fair Value of Financial Instruments
Carrying Value	290,250	329,844
Fair Value	290,250	329,844
Other
Fair Value of Financial Instruments
Carrying Value	5,316	10,243
Fair Value	$ 5,316	$ 10,243