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Stock-Based Compensation - Deferred Stock Units, DSUs (Details) - DSUs - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Units |  |  |
Balance outstanding at the beginning of the period (in shares) | 331,223 |  |
Granted (in shares) | 58,861 |  |
Converted to Common Stock (in shares) | (47,378) |  |
Balance outstanding at the end of the period (in shares) | 342,706 | 331,223 |
Weighted Average Grant Date Fair Value per Unit |  |  |
Balance outstanding at the beginning of the period, (in usd per share) | $ 23.98 |  |
Granted (in usd per share) | 35.59 | $ 34.84 | $ 33.43
Vested (in usd per share) | 26.87 |  |
Balance outstanding at the end of the period (in usd per share) | $ 25.37 | $ 23.98 |
Aggregate intrinsic value for DSUs outstanding | $ 13,000,000 | $ 13,000,000 | $ 13,000,000
Aggregate intrinsic values for DSUs converted to common stock during the period | $ 2,000,000 | $ 2,000,000 | $ 6,000,000
Weighted average grant date fair value (in usd per share) | $ 35.59 | $ 34.84 | $ 33.43

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- Definition The total accumulated difference between fair values of underlying shares on dates of conversion and conversion price on units which were converted into shares during the reporting period under the plan. + References No definition available. + Details
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- Definition As of the balance sheet date, the total dollar difference between fair values of the underlying shares reserved for issuance and exercise prices of fully vested equity awards other than options. + References No definition available. + Details
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- Definition Share-Based Compensation Arrangement By Share-Based Payment Award, Equity Instruments Other Than Options, Vested Number [Roll Forward] + References No definition available. + Details
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- Definition The weighted-average fair value of vested awards on equity-based plans excluding option plans. + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options Vested, Weighted Average Grant Date Fair Value [Abstract] + References No definition available. + Details
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- Definition 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 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)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition 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 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)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + References 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)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition 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. + References 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)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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