Fair Value Measurements (Fair Value of Senior Notes) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Senior Notes due 2021 at 4.875% [Member]
Carrying amount of senior notes	$ 497,701	$ 497,525
Estimated fair value of senior notes	548,605	524,434
Senior Notes due 2021 at 4.625% [Member]
Carrying amount of senior notes	299,624	299,598
Estimated fair value of senior notes	324,387	310,141
Senior Notes due 2022 at 3.800% [Member]
Carrying amount of senior notes	299,059	299,011
Estimated fair value of senior notes	306,063	290,453
Senior Notes due 2023 at 3.850% [Member]
Carrying amount of senior notes	299,978	299,976
Estimated fair value of senior notes	$ 306,593	$ 289,362

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 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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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