Summary of Significant Accounting Policies - Taxes and Foreign Currency (Details) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)
Income Taxes
Loss on trustee taxes	$ (52,600)	$ 16,200	$ (52,627)	$ 16,194
Penalties assessed				0
Expense (income) due to contingent liability or elimination of contingent liability	$ (52,600)	$ 16,200	$ (52,627)	$ 16,194
Number of wholly owned subsidiaries which are taxable for federal and state income tax purposes \| item			1

No definition available.

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- Definition

Represents the number of wholly owned subsidiaries of the entity which are taxable for federal and state income tax purposes.

+ References

No definition available.

+ Details

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- Definition

The amount of estimated penalties recognized in the period arising from income tax examinations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32718-109319

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- Definition

Amount of increase (decrease) to previously recorded tax expense. Includes, but is not limited to, significant settlements of income tax disputes, and unusual tax positions or infrequent actions taken by the entity, including tax assessment reversal, and IRS tax settlement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 25
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=115928272&loc=d3e28511-109314

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 25
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=115928272&loc=d3e28446-109314

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