Fair Value Measurements (Additional Information) (Details) (USD $)	12 Months Ended
Dec. 31, 2014 Impaired_Asset_And_Liabilty	Dec. 31, 2013 Impaired_Asset_And_Liabilty	Dec. 31, 2012 Impaired_Asset_And_Liabilty
Financial Assets and Liabilities Fair Value Measurement
Number of non-financial assets and liabilities impaired	0	0	0
Fair value assets Level 1 to Level 2 transfers amount	$ 0	$ 0	$ 0
Fair value liabilities Level 1 to Level 2 transfers amount	0	0	0
Fair value assets Level 2 to Level 1 transfers amount	0	0	0
Fair value liabilities Level 2 to Level 1 transfers amount	0	0	0
Other assets or liabilities required to be recognized at fair value on a non‑recurring basis	28,932,000us-gaap_LiabilitiesFairValueDisclosure	57,896,000us-gaap_LiabilitiesFairValueDisclosure
Nonrecurring Basis
Financial Assets and Liabilities Fair Value Measurement
Other assets or liabilities required to be recognized at fair value on a non‑recurring basis		$ 0	$ 0

Represents the number of non-financial assets and liabilities that were impaired during the period.

No definition available.

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- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bb)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

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- Definition

Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

+ Details

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- Definition

Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

+ Details

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- Definition

Amount of transfers of liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

+ Details

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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