Fair Value Measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value of Financial Instruments
Carrying Value	$ 4,255,332	$ 3,983,490
Fair Value	4,633,873	4,251,473
Subsidiary preferred stock, carrying value		69,188
Subsidiary preferred stock, fair value		69,000
2.35% senior notes due September 2016
Fair Value of Financial Instruments
Carrying Value	349,870	349,820
Fair Value	357,242	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	948,900	969,928
Fair Value	1,074,186	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	426,835	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	698,177	697,947
Fair Value	768,607	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	698,328	698,148
Fair Value	755,300	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	348,861	348,765
Fair Value	380,114	349,731
Interest rate on senior notes due (as a percent)	5.10%
Revolving Credit Facility
Fair Value of Financial Instruments
Carrying Value	100,000	170,000
Fair Value	100,000	170,000
Commercial paper
Fair Value of Financial Instruments
Carrying Value	771,374	329,844
Fair Value	771,374	329,844
Other
Fair Value of Financial Instruments
Carrying Value	215	10,243
Fair Value	$ 215	$ 10,243