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Summary of Significant Accounting Policies Stock-Based Compensation (Details) - $ / shares | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of shares authorized for issuance | 34,300,000 |  |
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 |  |
Non-qualified stock options granted (in shares) | 794,764 | 516,475 | 899,500
Estimated fair value per non-qualified stock option (in dollars per share) | $ 5.14 | $ 5.29 | $ 4.18
Risk-free interest rate, minimum (as a percent) | 0.40% | 0.10% | 0.10%
Risk-free interest rate, maximum (as a percent) | 2.20% | 2.10% | 3.00%
Dividend yield (as a percent) | 4.00% | 3.70% | 3.80%
Expected volatility (as a percent) | 18.10% | 18.00% | 18.00%
Expected life (in years) | 6 years 1 month | 5 years 10 months | 5 years 10 months
Restricted stock |  |  |
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% |  |
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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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum risk-free interest rate assumption that is used in valuing an option on its own shares. + References No definition available. + Details
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- Definition The minimum risk-free interest rate assumption that is used in valuing an option on its own shares. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Gross number of share options (or share units) granted during the period. + References No definition available. + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: 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.D.2) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 + Details
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- Definition Minimum Exercise Price of Stock Option as a Percent of Common Stock Fair Value on the Grant Date + References No definition available. + Details
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