Stock-Based Compensation, Stock Option Activity (Details) - USD ($)	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair value assumptions of stock options
Dividend yield	2.50%	2.00%
Risk-free interest rate	2.60%	2.00%
Weighted average expected volatility	22.00%	25.00%
Weighted average expected life	5 years 9 months	5 years 6 months
Weighted average per share fair value of stock options (in usd per share)	$ 4.47	$ 5.30
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Stock Options Shares Outstanding, Beginning balance (in shares)	5,118,687
Granted (in shares)	938,740
Exercised (in shares)	(888,661)
Forfeited or expired (in shares)	(38,613)
Stock Options Shares Outstanding, Ending balance (in shares)	5,500,204		5,118,687
Options exercisable (in shares)	3,287,488
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Stock Options Outstanding, Weighted Average Exercise Price, Beginning balance (in usd per share)	$ 18.02
Granted, Weighted Average Exercise Price (in usd per share)	$ 24.50
Shares used in acquisition	$ 370,051
Exercised, Weighted Average Exercise Price (in usd per share)	$ 16.69
Forfeited or expired, Weighted Average Exercise Price (in usd per share)	24.21
Stock Options Outstanding, Weighted Average Exercise Price, Ending balance (in usd per share)	$ 19.05		$ 18.02
Ex Share Price at Acquisition	$ 14.35
Options Exercisable, Weighted Average Exercise Price (in usd per share)	$ 16.80
Stock Options Outstanding, Weighted Average Remaining Contractual Term	6 years 8 months 8 days		6 years 5 months 23 days
Options exercisable, Weighted Average Remaining Contractual Term	5 years 4 months 20 days
Stock Options Outstanding, Aggregate Intrinsic Value	$ 45,418,000		$ 38,028,000
Options exercisable, Aggregate Intrinsic Value	$ 34,551,000

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Per share fair value of stock options.

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Shares used in acquisition

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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.

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The estimated measure of the 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.

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

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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

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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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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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