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Stock-Based Compensation Plans 2 (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Number of Options |  |  |
Exercised (in shares) | (52,936) | (12,546) | (66,458)
Options outstanding at the end of the period (in shares) | 69,202 |  |
Additional disclosure |  |  |
Cash proceeds from the exercise of stock options | $ 909,000 | $ 235,000 | $ 1,198,000
Tax benefit for the tax deduction from awards exercised | 0 | $ (581,000) | $ (877,000)
Unrecognized compensation cost | $ 1,059,000 |  |
Maximum |  |  |
Additional disclosure |  |  |
Percentage of target amount of performance shares | 200.00% |  |
Minimum |  |  |
Additional disclosure |  |  |
Percentage of target amount of performance shares | 0.00% |  |
Weighted Average [Member] |  |  |
Additional disclosure |  |  |
Expected recognition period for unrecognized compensation cost | 1 year 6 months 30 days |  |
Stock options |  |  |
Number of Options |  |  |
Options Outstanding at the beginning of the period (in shares) | 136,560 |  |
Granted (in shares) | 0 | 0 | 0
Exercised (in shares) | (52,936) |  |
Forfeited or expired (in shares) | (14,422) |  |
Options outstanding at the end of the period (in shares) | 69,202 | 136,560 |
Options exercisable at the end of the period (in shares) | 69,202 |  |
Weighted Average Exercise Price |  |  |
Options outstanding at the beginning of the period (in dollars per share) | $ 17.27 |  |
Exercised (in dollars per share) | 17.18 |  |
Forfeited or expired (in dollars per share) | 19.50 |  |
Options outstanding at the end of the period (in dollars per share) | 16.87 | $ 17.27 |
Options exercisable at the end of the period (in dollars per share) | $ 16.87 |  |
Additional disclosure |  |  |
Weighted Average Remaining Contractual Term, Options outstanding | 1 year 4 months 6 days |  |
Weighted Average Remaining Contractual Term, Options Exercisable | 1 year 4 months 6 days |  |
Aggregate Intrinsic Value, Options outstanding | $ 1,167,405 |  |
Aggregate Intrinsic Value, Options Exercisable | 1,167,405 |  |
Total intrinsic value of options exercised | 1,718,000 | $ 308,000 | $ 1,457,000
Tax benefit for the tax deduction from awards exercised | (1,000,000) | $ (581,000) | (877,000)
Restricted Stock Units |  |  |
Additional disclosure |  |  |
Unrecognized compensation cost related to performance awards | $ 824,000 |  |
Number of Restricted/Performance Share Units |  |  |
Restricted share units at the beginning of the period (in shares) | 107,729 |  |
Granted (in shares) | 41,755 |  |
Vested (in shares) | (40,695) |  |
Forfeited (in shares) | (1,502) |  |
Restricted share units at the end of the period (in shares) | 107,287 | 107,729 |
Weighted Average Grant-Date Value |  |  |
Restricted share units at the beginning of the period (in dollars per share) | $ 30.83 |  |
Granted (in dollars per share) | 44.46 |  |
Vested (in dollars per share) | 39.32 |  |
Forfeited (in dollars per share) | 42.32 |  |
Restricted share units at the end of the period (in dollars per share) | $ 32.75 | $ 30.83 |
Restricted Stock Units | Weighted Average [Member] |  |  |
Additional disclosure |  |  |
Expected recognition period for unrecognized compensation cost | 1 year 4 months 10 days |  |
Performance awards |  |  |
Additional disclosure |  |  |
Vesting period | 3 years |  |
Period to meet the performance goals | 3 years |  |
Number of Restricted/Performance Share Units |  |  |
Restricted share units at the beginning of the period (in shares) | 102,203 |  |
Granted (in shares) | 31,558 |  |
Performance criteria adjustment (in shares) | 9,332 |  |
Vested (in shares) | (45,214) |  |
Restricted share units at the end of the period (in shares) | 97,879 | 102,203 |
Weighted Average Grant-Date Value |  |  |
Restricted share units at the beginning of the period (in dollars per share) | $ 35.25 |  |
Granted (in dollars per share) | 44.10 |  |
Performance criteria adjustment (in dollars per share) | 43.40 |  |
Vested (in dollars per share) | 29.60 |  |
Restricted share units at the end of the period (in dollars per share) | $ 41.49 | $ 35.25 |
GSWC |  |  |
Additional disclosure |  |  |
Tax benefit for the tax deduction from awards exercised | $ 0 | $ (501,000) | (872,000)
GSWC | Stock options |  |  |
Additional disclosure |  |  |
Tax benefit for the tax deduction from awards exercised | $ (1,000,000) | $ (501,000) | $ (872,000)

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- Definition Unrecognized cost of unvested share-based compensation awards. + 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 (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition 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. + 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 (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Amount of cash inflow from realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 20 -Section 55 -Paragraph 24 -URI http://asc.fasb.org/extlink&oid=96864182&loc=d3e11374-113907 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting 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 (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event. + 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 (c)(2)(iii)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + 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 (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units. + 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 (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting 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 (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement. + 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 (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average price of options that were either forfeited or expired. + 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 (c)(1)(iv)(3)-(4) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan. + 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 (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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