Debt Obligations ( 4.00% And 5.55% Senior Unsecured Notes, Credit Facility) (Narrative) (Details) (USD $)	9 Months Ended		1 Months Ended	9 Months Ended					6 Months Ended	9 Months Ended	6 Months Ended	9 Months Ended	3 Months Ended					9 Months Ended	12 Months Ended	1 Months Ended		9 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Jan. 31, 2010 Senior Unsecured Notes [Member]	Sep. 30, 2011 Senior Unsecured Notes [Member]	Sep. 30, 2010 Senior Unsecured Notes [Member]	Sep. 30, 2011 Senior Unsecured Notes [Member] 2010 Credit Facility [Member]	Jan. 31, 2010 Tranche A Term Loan [Member]	Jan. 31, 2010 Tranche X Term Loan [Member]	Jun. 30, 2011 Quarterly Payments [Member] 2010 Credit Facility [Member]	Sep. 30, 2011 Quarterly Payments [Member] 2010 Credit Facility [Member]	Jun. 30, 2011 Optional Payments [Member] 2010 Credit Facility [Member]	Sep. 30, 2011 2016 Term Loan [Member]	Dec. 31, 2011 Revolving Credit Facility [Member] 2011 Credit Facility [Member]	Sep. 30, 2011 Revolving Credit Facility [Member] 2011 Credit Facility [Member]	Sep. 30, 2011 Revolving Credit Facility [Member] 2010 Credit Facility [Member]	Sep. 30, 2011 4.00% Senior Notes Due 2015 [Member]	Sep. 30, 2011 5.55% Senior Notes Due 2020 [Member]	Sep. 30, 2011 Clearinghouse Credit Facilities [Member]	Dec. 31, 2010 Clearinghouse Credit Facilities [Member]	Dec. 31, 2010 5.25% Senior Unsecured Notes [Member]	Sep. 30, 2011 5.25% Senior Unsecured Notes [Member]	Sep. 30, 2011 2011 Credit Facility [Member]	Sep. 30, 2011 2010 Credit Facility [Member]	Sep. 30, 2010 2010 Credit Facility [Member]	Sep. 30, 2011 2010 Credit Facility [Member]	Sep. 30, 2010 2010 Credit Facility [Member]	Jan. 31, 2010 2010 Credit Facility [Member]
Debt Instrument [Line Items]
Term loans quarterly payment									$ 70,000,000	$ 35,000,000
Term loans, optional payment											50,000,000
Senior unsecured notes issued			1,000,000,000																	370,000,000
Debt instrument, interest rate, stated percentage																4.00%	5.55%				5.25%
Debt instrument, maturity date																						Sep. 19, 2016
Total credit facility							350,000,000	350,000,000				450,000,000
Credit facility, additional borrowings													250,000,000
Credit facility, borrowing capacity																		433,000,000	440,000,000			1,200,000,000					950,000,000
Credit facility, remaining capacity														750,000,000	250,000,000							500,000,000
Proceed from term loan												450,000,000
Quarterly principal repayment percentage																						2.50%
Debt instrument, face amount																399,000,000	598,000,000
Debt instrument, principal outstanding				450,000,000		450,000,000										400,000,000	600,000,000				367,000,000
Debt issuance costs				8,000,000																	3,000,000	5,000,000	13,000,000		13,000,000
Amortization expenses recorded as additional interest expense	5,000,000	5,000,000		1,000,000	1,000,000																		1,000,000	1,000,000	3,000,000	3,000,000
Write off of deferred debt issuance cost	6,000,000	37,000,000																					6,000,000		6,000,000
Line of credit facility, available liquidity																		192,000,000	196,000,000
Credit facility satisfy regulatory requirements																		241,000,000	244,000,000
Cash on hand used														96,000,000
Credit facility, available capacity														$ 750,000,000	$ 250,000,000							$ 500,000,000