Stockholders' Equity (Nonvested Stock Awards) (Details) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Weighted Average Grant-Date Fair Value
Unvested at beginning of period (in dollars per share)	$ 8.42	$ 22.62	$ 37.22
Granted (in dollars per share)	3.66	7.56	22.53
Vested (in dollars per share)	7.22	23.96	27.55
Forfeited (in dollars per share)	20.02	18.47	29.50
Unvested at end of period (in dollars per share)	$ 6.99	$ 8.42	$ 22.62
Restricted Stock Units
Unvested Stock Awards
Unvested as of beginning of period (in shares)	2,660,236	1,466,135	779,912
Granted (in shares)	634,570	2,578,866	2,872,408
Vested (in shares)	(1,363,152)	(854,998)	(2,077,253)
Forfeited (in shares)	(106,417)	(529,767)	(108,932)
Unvested as of end of period (in shares)	1,825,237	2,660,236	1,466,135

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

+ References

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-Publisher FASB
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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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-Publisher FASB
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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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