Share-Based Transactions - Nonemployee Share-Based Transactions, Valuation Assumptions (Details) - Nonemployee Shares [Member] - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Valuation Assumptions For Restricted Stock Units [Line Items]
Expected volatility, Minimum	18.00%	24.00%
Expected volatility, Maximum	34.00%	42.00%
Minimum [Member]
Valuation Assumptions For Restricted Stock Units [Line Items]
Quarterly dividends expected	$ 0.19	$ 0.17
Maximum [Member]
Valuation Assumptions For Restricted Stock Units [Line Items]
Quarterly dividends expected	$ 0.20	$ 0.18
Risk-free interest rates	3.00%	2.00%

Share based compensation arrangement by share based payment award fair value assumptions expected dividend rate range.

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- Definition

Valuation assumptions for restricted stock units.

+ References

No definition available.

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- Definition

The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

No definition available.

+ Details

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- Definition

The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

No definition available.

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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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