Debt (Narrative) (Details) (USD $)	1 Months Ended	6 Months Ended		3 Months Ended					6 Months Ended
	Feb. 28, 2011 interest_rate_swaps	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Senior Debt Obligations Due 2017 [Member]	Jun. 30, 2012 Senior Debt Obligations Due 2012 [Member]	Jun. 30, 2012 Senior Debt Obligations Due 2014 [Member]	Jun. 27, 2011 Senior Debt Obligations Due 2014 [Member]	Jun. 30, 2012 Senior Debt Obligations Due 2015 [Member]	Jun. 30, 2012 September 2011 Debt Obligation [Member]	Oct. 13, 2011 September 2011 Debt Obligation [Member]
Debt instrument, interest rate, stated percentage				2.30%	6.25%	5.375%	5.375%	5.75%
Extinguishment of Debt, Amount					$ 250,000,000
Line of Credit Facility Term									5 years
Line of credit facility, current borrowing capacity										1,000,000,000
Number of interest rate swaps (in interest rate swaps)	2
Interest rate swap for hedging	125,000,000
Interest rate swap maturity, in years	3.5
Floating rate three-month LIBOR plus fixed spread interest percentage	3.726%
Interest rate swaps, fair value adjustment	0
Swap agreements, effectiveness recognized during the period		0
5.375% senior notes due in 2014, hedged value						250,000,000
Revolving credit facility, amount outstanding		0
Proceeds from issuance of debt		$ 248,000,000		$ 250,000,000
Term of interest rate swap net settlement (in months)		6 months