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Stock-Based Compensation - Narrative (Details) | 12 Months Ended
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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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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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