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Summary of Significant Accounting Policies Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of shares authorized for issuance | 34,300,000 |  |
Cumulative-effect of adoption of ASU 2016-09 |  | $ 15.7 |
Stock options |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period (in years) | 3 years |  |
Minimum exercise price of stock option as a percent of common stock fair value on the grant date | 100.00% |  |
Period after the grant date during which stock options can't be exercised (in months) | 6 months |  |
Maximum term of awards (in years) | 10 years |  |
Stock options granted (in shares) | 552,215 | 794,764 | 516,475
Estimated weighted-average fair value per stock option (in dollars per share) | $ 7.45 | $ 5.14 | $ 5.29
Risk-free interest rate, minimum (as a percent) | 0.70% | 0.40% | 0.10%
Risk-free interest rate, maximum (as a percent) | 2.50% | 2.20% | 2.10%
Dividend yield (as a percent) | 3.50% | 4.00% | 3.70%
Expected volatility (as a percent) | 19.00% | 18.10% | 18.00%
Expected life (in years) | 6 years 9 months | 6 years 1 month | 5 years 10 months
Restricted stock | Employees |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period (in years) | 3 years |  |
Percentage to vest each year after grant date | 33.00% |  |
Restricted stock | Directors | Granted prior to January 1, 2017 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period (in years) | 3 years |  |
Percentage to vest each year after grant date | 33.00% |  |
Restricted stock | Directors | Granted after January 1, 2017 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period (in years) | 1 year |  |
Performance units |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period (in years) | 3 years |  |
Performance units | Minimum |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Performance units, payout ratio (as a percent) | 0.00% |  |
Performance units | Maximum |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Performance units, payout ratio (as a percent) | 175.00% |  |

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