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Debt Obligations (Narrative) (Details) (USD $)
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3 Months Ended | 12 Months Ended | 3 Months Ended | 12 Months Ended | 3 Months Ended | 12 Months Ended | 3 Months Ended | 1 Months Ended | 3 Months Ended | 1 Months Ended | 3 Months Ended | |||||||||||||||
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Mar. 31, 2013
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Mar. 31, 2012
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Dec. 31, 2012
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Mar. 31, 2013
Liability Component [Member]
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Dec. 31, 2012
Liability Component [Member]
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Mar. 31, 2013
Senior Unsecured Notes [Member]
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Jan. 31, 2010
Senior Unsecured Notes [Member]
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Mar. 31, 2013
Equity Component [Member]
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Dec. 31, 2012
Equity Component [Member]
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Mar. 31, 2013
2.50% convertible senior notes due August 15, 2013 [Member]
item
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Mar. 31, 2012
2.50% convertible senior notes due August 15, 2013 [Member]
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Mar. 31, 2008
2.50% convertible senior notes due August 15, 2013 [Member]
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Dec. 31, 2012
2.50% convertible senior notes due August 15, 2013 [Member]
item
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Dec. 31, 2012
4.00% Senior Notes Due 2015 [Member]
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Mar. 31, 2013
4.00% Senior Notes Due 2015 [Member]
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Mar. 31, 2013
5.55% Senior Notes Due 2020 [Member]
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Dec. 31, 2012
5.55% Senior Notes Due 2020 [Member]
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Dec. 31, 2010
5.25% Senior Unsecured Notes Due January 16, 2018 [Member]
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Mar. 31, 2013
5.25% Senior Unsecured Notes Due January 16, 2018 [Member]
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Oct. 31, 2011
2011 Credit Facility [Member]
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Sep. 30, 2011
2011 Credit Facility [Member]
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Mar. 31, 2013
2011 Credit Facility [Member]
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Sep. 30, 2011
2011 Credit Facility [Member]
2016 Term Loan [Member]
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Sep. 30, 2011
2011 Credit Facility [Member]
Revolving Credit Facility [Member]
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Mar. 31, 2013
Clearinghouse Credit Facilities [Member]
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Dec. 31, 2012
Clearinghouse Credit Facilities [Member]
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| Schedule of Capitalization, Long-term Debt [Line Items] | ||||||||||||||||||||||||||
| Debt instrument, principal outstanding | $ 93,000,000 | $ 93,000,000 | $ 475,000,000 | $ 400,000,000 | $ 600,000,000 | $ 370,000,000 | ||||||||||||||||||||
| Debt instrument, interest rate | 2.50% | 2.50% | 2.50% | 4.00% | 5.55% | 5.25% | ||||||||||||||||||||
| Debt instrument, maturity date | Aug. 15, 2013 | Aug. 15, 2013 | Jan. 15, 2015 | Jan. 15, 2020 | Jan. 16, 2018 | Sep. 19, 2016 | ||||||||||||||||||||
| Debt instrument, frequency of payment | semiannually | |||||||||||||||||||||||||
| Conversion of stock issued, conversion rate | 18.5249 | 18.4504 | ||||||||||||||||||||||||
| Convertible notes, conversion price per share of common stock | $ 53.98 | $ 54.20 | ||||||||||||||||||||||||
| Debt instrument convertible, number of equity instruments | 1,722,508 | 1,715,517 | ||||||||||||||||||||||||
| Percentage of principal amount of notes purchased if fundamental change is applied, maximum | 100.00% | |||||||||||||||||||||||||
| Debt instrument, unamortized discount | 1,000,000 | 2,000,000 | 1,000,000 | 2,000,000 | ||||||||||||||||||||||
| Debt instrument, convertible, effective interest rate | 6.53% | 6.53% | ||||||||||||||||||||||||
| Debt instrument, equity component of the convertible debt | 71,000,000 | 71,000,000 | 39,000,000 | 39,000,000 | ||||||||||||||||||||||
| Debt instrument, face amount | 1,000,000,000 | |||||||||||||||||||||||||
| Debt issuance costs | 8,000,000 | 5,000,000 | ||||||||||||||||||||||||
| Debt instrument, aggregate principal, less unamortized debt discount | 399,000,000 | 598,000,000 | 368,000,000 | |||||||||||||||||||||||
| Debt instrument, interest rate, maximum | 7.25% | |||||||||||||||||||||||||
| Percentage of aggregate principal owed upon triggering event | 101.00% | |||||||||||||||||||||||||
| Accretion of debt discounts | 1,000,000 | 1,000,000 | ||||||||||||||||||||||||
| Equity component in convertible debt | 39,000,000 | 39,000,000 | ||||||||||||||||||||||||
| Amortization expense | 1,000,000 | 1,000,000 | ||||||||||||||||||||||||
| Credit facility, borrowing capacity | 1,200,000,000 | 1,200,000,000 | 308,000,000 | 310,000,000 | ||||||||||||||||||||||
| Line of credit, outstanding amount | 450,000,000 | 750,000,000 | 0 | 0 | ||||||||||||||||||||||
| Line of credit facility, frequency of payments | quarterly | |||||||||||||||||||||||||
| Credit facility, additional borrowings | 250,000,000 | |||||||||||||||||||||||||
| Credit facility, available capacity | 624,000,000 | |||||||||||||||||||||||||
| Line of credit facility, quarterly principal payment percentage | 2.50% | |||||||||||||||||||||||||
| Line of credit facility, quarterly principal payment | 11,000,000 | |||||||||||||||||||||||||
| Repayment of loan | 450,000,000 | |||||||||||||||||||||||||
| Line of credit facility, available liquidity | 216,000,000 | 217,000,000 | ||||||||||||||||||||||||
| Credit facility satisfy regulatory requirements | $ 92,000,000 | $ 93,000,000 | ||||||||||||||||||||||||