| **Consolidated Statements Of Capitalization - USD ($) $ in Thousands**          | Jun. 30, 2018 | Dec. 31, 2017 |
|---------------------------------------------------------------------------------|---------------|---------------|
| **Stockholders' equity:**                                                       |          |          |
| Common stock, $.50 par value                                                    | $ 90,486      | $ 90,350      |
| Capital in excess of par value                                                  | 811,763       | 807,135       |
| Retained earnings                                                               | 1,177,874     | 1,132,556     |
| Treasury stock, at cost                                                         | (75,771)      | (73,280)      |
| Accumulated other comprehensive income                                          |          | 860           |
| Total stockholders' equity                                                      | 2,004,352     | 1,957,621     |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 1,553,676     | 1,462,900     |
| Total long-term debt                                                            | 2,320,903     | 2,143,127     |
| Current portion of long-term debt                                               | 118,540       | 113,769       |
| Long-term debt, excluding current portion                                       | 2,202,363     | 2,029,358     |
| Less: debt issuance costs                                                       | 21,002        | 21,605        |
| Long-term debt, excluding current portion, net of debt issuance costs           | 2,181,361     | 2,007,753     |
| Total capitalization                                                            | 4,185,713     | 3,965,374     |
| Long-Term Debt Of Subsidiaries 0.00% To 0.99% [Member]                          |          |          |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 3,876         | 4,196         |
| Long-Term Debt Of Subsidiaries 1.00% To 1.99% [Member]                          |          |          |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 12,343        | 12,914        |
| Long-Term Debt Of Subsidiaries 2.00% To 2.99% [Member]                          |          |          |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 18,377        | 19,254        |
| Long-Term Debt Of Subsidiaries 3.00% To 3.99% [Member]                          |          |          |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 498,841       | 475,232       |
| Long-Term Debt Of Subsidiaries 4.00% To 4.99% [Member]                          |          |          |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 706,431       | 631,599       |
| Long-Term Debt Of Subsidiaries 5.00% To 5.99% [Member]                          |          |          |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 205,311       | 205,578       |
| Long-Term Debt Of Subsidiaries 6.00% To 6.99% [Member]                          |          |          |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 44,000        | 44,000        |
| Long-Term Debt Of Subsidiaries 7.00% To 7.99% [Member]                          |          |          |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 31,955        | 32,335        |
| Long-Term Debt Of Subsidiaries 8.00% To 8.99% [Member]                          |          |          |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 5,842         | 6,092         |
| Long-Term Debt Of Subsidiaries 9.00% To 9.99% [Member]                          |          |          |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 20,700        | 25,700        |
| Long-Term Debt Of Subsidiaries 10.00% To 10.99% [Member]                        |          |          |
| **Long-term debt:**                                                             |          |          |
| Long-term debt of subsidiaries                                                  | 6,000         | 6,000         |
| Revolving Credit Agreement, Due 2021 [Member]                                   |          |          |
| **Long-term debt:**                                                             |          |          |
| Notes payable to bank under revolving credit agreement, variable rate, due 2021 | 157,000       | 60,000        |
| Bank notes at 2.48% and 3.5% due 2019 and 2020 [Member]                         |          |          |
| **Long-term debt:**                                                             |          |          |
| Unsecured notes payable                                                         | 100,000       | 100,000       |
| Notes at 3.01% and 3.59% due 2027 and 2041 [Member]                             |          |          |
| **Long-term debt:**                                                             |          |          |
| Unsecured notes payable                                                         | 245,000       | 245,000       |
| Notes Ranging From 4.62% To 4.87%, due 2018 through 2024 [Member]               |          |          |
| **Long-term debt:**                                                             |          |          |
| Unsecured notes payable                                                         | 122,800       | 122,800       |
| Notes Ranging From 5.20% To 5.95%, due 2018 through 2037 [Member]               |          |          |
| **Long-term debt:**                                                             |          |          |
| Unsecured notes payable                                                         | $ 142,427     | $ 152,427     |

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| - Definition
Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.
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Value received from shareholders in common stock-related transactions that are in excess of par value or stated value and amounts received from other stock-related transactions. Includes only common stock transactions (excludes preferred stock transactions). May be called contributed capital, capital in excess of par, capital surplus, or paid-in capital.
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This element represents the total consolidated (as applicable) capitalization of the entity which is comprised of its long-term debt and equity instruments. The table may be detailed by subsidiary (legal entity) and include information by type of debt or equity detailed by instrument.
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Aggregate par or stated value of issued nonredeemable common stock (or common stock redeemable solely at the option of the issuer). This item includes treasury stock repurchased by the entity. Note: elements for number of nonredeemable common shares, par value and other disclosure concepts are in another section within stockholders' equity.
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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.
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Amount, after accumulated amortization, of debt issuance costs classified as noncurrent. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.
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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.
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Including the current and noncurrent portions, the carrying value as of the balance sheet date of notes payable to banks, excluding mortgage notes, initially due beyond one year or beyond the operating cycle if longer.
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The cumulative amount of the reporting entity's undistributed earnings or deficit.
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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.
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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.
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The amount allocated to treasury stock. Treasury stock is common and preferred shares of an entity that were issued, repurchased by the entity, and are held in its treasury.
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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).
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Debt Instrument Carrying Amount Noncurrent
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| - Details**Name:**us-gaap_DebtInstrumentAxis=wtr_LongTermDebtOfSubsidiariesEightPointZeroZeroPercentageToEightPointNineNinePercentageMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentAxis=wtr_LongTermDebtOfSubsidiariesNinePointZeroZeroPercentageToNinePointNineNinePercentageMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentAxis=wtr_LongTermDebtOfSubsidiariesTenPointZeroZeroPercentageToTenPointNineNinePercentageMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentAxis=wtr_RevolvingCreditAgreementDueBMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentAxis=wtr_BankNoteAtOnePointNineTwoOnePercentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentAxis=wtr_NotesRangesBMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentAxis=wtr_NotesRangingFromFourPointSixTwoPercentageToFourPointEightSevenPercentageMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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