Senior Bank Debt (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2012	Dec. 31, 2011
Amended credit facility
Senior Bank Debt
Maximum borrowing capacity	$ 1,000.0
Revolving credit facility
Senior Bank Debt
Maximum borrowing capacity	750.0
Principal amount maturing on November, 2016	720.0
Term of debt (in years)	5 years
Remaining principal amount maturing on January, 2015	30.0
Option to increase the borrowing capacity under revolving credit facility	150.0
Line of credit facility amount outstanding	445.0	250.0
Term loan
Senior Bank Debt
Maximum borrowing capacity	250.0
Term of debt (in years)	5 years
Option to increase the borrowing capacity under revolving credit facility	$ 250.0

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant