Stock-Based Compensation - Activity and Status of Unvested Shares and Performance Units (Details) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted Stock
Units
Beginning balance (in shares)	459,784	703,796	1,087,186
Granted (in shares)	404,779	247,521	190,557
Vested (in shares)	(420,970)	(470,019)	(571,481)
Canceled (in shares)	(948)	(21,514)	(2,466)
Ending balance (in shares)	442,645	459,784	703,796
Weighted-Average Grant-Date Fair Value
Beginning balance (in dollars per share)	$ 33.67	$ 32.93	$ 33.64
Granted (in dollars per share)	24.70	35.50	29.53
Vested (in dollars per share)	31.61	32.88	32.74
Canceled (in dollars per share)	29.91	34.16	33.38
Ending balance (in dollars per share)	$ 27.44	$ 33.67	$ 32.93
Performance units
Units
Beginning balance (in shares)	608,679	318,549	0
Granted (in shares)	571,978	481,215	318,549
Vested (in shares)	(409,225)	(191,085)	0
Canceled (in shares)	0	0	0
Ending balance (in shares)	771,432	608,679	318,549
Weighted-Average Grant-Date Fair Value
Beginning balance (in dollars per share)	$ 32.70	$ 28.41	$ 0
Granted (in dollars per share)	23.49	35.74	28.41
Vested (in dollars per share)	30.42	30.37	0
Canceled (in dollars per share)	0	0	0
Ending balance (in dollars per share)	$ 27.08	$ 32.70	$ 28.41

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)
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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).

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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.

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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.

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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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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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