Senior Notes (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2012	Dec. 31, 2011
Schedule of Carrying Amount of Senior Notes [Line Items]
Senior notes	$ 744,288	$ 744,108
5.375% Senior Notes Due 2014 [Member]
Schedule of Carrying Amount of Senior Notes [Line Items]
Senior notes	249,483	249,438
Interest rate	5.375%
Maturity date	2014
5.375% Senior Notes Due 2015 [Member]
Schedule of Carrying Amount of Senior Notes [Line Items]
Senior notes	249,866	249,857
Interest rate	5.375%
Maturity date	2015
5.625% Senior Notes Due 2020 [Member]
Schedule of Carrying Amount of Senior Notes [Line Items]
Senior notes	$ 244,939	$ 244,813
Interest rate	5.625%
Maturity date	2020

Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 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 210.5-02.22(a)(2))

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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration