Fair Value Measurements (Fair Value of Senior Notes) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Senior Notes due 2021 at 4.875% [Member]
Carrying amount of senior notes	$ 497,349	$ 497,173
Estimated fair value of senior notes	531,007	559,870
Senior Notes due 2021 at 4.625% [Member]
Carrying amount of senior notes	299,572	299,545
Estimated fair value of senior notes	313,321	331,224
Senior Notes due 2022 at 3.800% [Member]
Carrying amount of senior notes	298,963	298,916
Estimated fair value of senior notes	295,167	313,890
Senior Notes due 2023 at 3.850% [Member]
Carrying amount of senior notes	299,976	0
Estimated fair value of senior notes	$ 291,803	$ 0

Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: 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)

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

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