Stock-Based Compensation - Narrative (Details)	12 Months Ended
Dec. 31, 2017 USD ($) plan shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock-based compensation plans \| plan	2
Options expiration period	10 years
Stock option awards granted \| shares	0	0
Stock compensation expense	$ 3,900,000	$ 4,400,000	$ 4,000,000
Tax benefits from stock-based compensation	2,600,000	2,100,000	1,400,000
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	0	40,000	480,000
Tax benefits from stock-based compensation	0	20,000	180,000
Intrinsic value of fully-vested stock options outstanding	15,400,000
Total intrinsic value of options exercised	6,300,000	5,600,000	3,900,000
Actual tax benefit realized for tax deduction	2,000,000	2,100,000	1,500,000
Proceeds from stock options exercised	2,400,000	4,700,000	3,400,000
Common stock redeemed for payment of stock option exercises, aggregate value	2,800,000	3,100,000	2,500,000
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	3,300,000	3,900,000	2,900,000
Tax benefits from stock-based compensation	$ 600,000	$ 1,500,000	$ 1,100,000
Stock issued during period, shares, restricted stock award \| shares	140,246
Unrecognized compensation cost related to nonvested restricted stock awards	$ 5,800,000
Unrecognized compensation cost related to nonvested restricted stock to be recognized over weighted average period (in years)	1 year 10 months 5 days
Restricted Stock \| Upon Achievement of Performance Goals
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award \| shares	35,634
Restricted Stock \| Upon Achievement of Defined Return on Equity Performance Goals
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award \| shares	17,817
Restricted Stock \| Upon Achievement of Defined Total Return to Shareholder Goals
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award \| shares	17,817
Restricted Stock \| Upon One-Third Annual Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award \| shares	104,612
Restricted Stock \| First Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage	33.33%
Restricted Stock \| Second Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage	33.33%
Restricted Stock \| Third Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage	33.33%
2006 Plan \| Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant \| shares	1,954,046

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Share-Based Compensation Arrangement By Share-Based Payment Award, Number Of Compensation Plans

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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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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.

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Amount of tax benefit from exercise of stock options granted under share-based compensation arrangement.

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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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

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Percentage of vesting of share-based compensation awards.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of shares issued during the period related to Restricted Stock Awards, net of any shares forfeited.

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