Share-Based Payments - Amounts for Employee Options Granted were Estimated on Date of Grant Using Modified Black-Scholes Option Pricing Model (Details) - Employee Stock Purchase Plan	12 Months Ended
Dec. 31, 2018 $ / shares
March 25, 2015 (1)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 8.17
Risk free interest rate	0.94%
Dividend yield	1.78%
Expected life (in years)	3
Share price volatility	16.00%
March 25, 2015 (2)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 7.08
Risk free interest rate	0.60%
Dividend yield	1.78%
Expected life (in years)	2
Share price volatility	16.00%
March 25, 2016 (1)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 7.97
Risk free interest rate	1.04%
Dividend yield	1.88%
Expected life (in years)	3
Share price volatility	4.21%
March 25, 2016 (2)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 7.00
Risk free interest rate	0.87%
Dividend yield	1.88%
Expected life (in years)	2
Share price volatility	2.44%
April 28, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 6.69
Risk free interest rate	1.44%
Dividend yield	1.83%
Expected life (in years)	3
Share price volatility	3.67%
April 28, 2017 (2)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 8.70
Risk free interest rate	1.44%
Dividend yield	1.83%
Expected life (in years)	3
Share price volatility	3.67%
April 28, 2017 (3)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 8.70
Risk free interest rate	1.26%
Dividend yield	1.83%
Expected life (in years)	2
Share price volatility	3.52%

Expected life.

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- Definition

Share price volatility.

+ References

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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=109197908&loc=d3e5070-113901

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ 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 (f)(2)(iii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

+ 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 (f)(2)(iv)
-URI http://asc.fasb.org/extlink&oid=109197908&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.

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