Stock Compensation Plans (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options outstanding (in shares)	0	0
RSAs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards other than options outstanding (in shares)	0	0
RSUs \| Ratably
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
RSUs \| Cliff \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
RSUs \| Cliff \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	5 years
Performance-Based RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards other than options outstanding (in shares)	411,290	424,299
Total unrecognized compensation cost	$ 26.0
Weighted average period to recognize unrecognized compensation cost	2 years 4 months 13 days
Performance-Based RSUs \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential for awards to vest (as a percent)	0.00%
Performance-Based RSUs \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential for awards to vest (as a percent)	200.00%
Performance-Based RSUs \| Cliff
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Time-Based RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards other than options outstanding (in shares)	1,214,611	1,166,580
Total unrecognized compensation cost	$ 46.8
Weighted average period to recognize unrecognized compensation cost	2 years 5 months 8 days

Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Target Award Available for Grant

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Number of options outstanding, including both vested and non-vested options.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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