Segment Information (Company's Segment Information, Continuing Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Operating revenues	$ 190,326us-gaap_Revenues	$ 182,672us-gaap_Revenues
Operations and maintenance expense	73,189	71,686
Depreciation	30,500us-gaap_Depreciation	30,981us-gaap_Depreciation
Operating income	71,167us-gaap_OperatingIncomeLoss	66,770us-gaap_OperatingIncomeLoss
Interest expense, net of AFUDC	17,483wtr_InterestExpenseNetOfAfudc	18,143wtr_InterestExpenseNetOfAfudc
Income tax expense (benefit)	4,594us-gaap_IncomeTaxExpenseBenefit	5,192us-gaap_IncomeTaxExpenseBenefit
Income (loss) from continuing operations	48,545	42,401
Capital expenditures	70,284	59,819
Regulated Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	180,564	178,199
Operations and maintenance expense	67,018	68,456
Depreciation	30,438	30,881
Operating income	68,624	66,419
Interest expense, net of AFUDC	16,720	16,804
Income tax expense (benefit)	4,367	6,087
Income (loss) from continuing operations	47,705	43,620
Capital expenditures	70,111	59,154
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	9,762	4,473
Operations and maintenance expense	6,171	3,230
Depreciation	62	100
Operating income	2,543	351
Interest expense, net of AFUDC	763	1,339
Income tax expense (benefit)	227	(895)
Income (loss) from continuing operations	840	(1,219)
Capital expenditures	$ 173	$ 665

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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Balance Type:	debit
Period Type:	duration

- Definition

This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses and taxes from ongoing operations before extraordinary items, and noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 45

-Paragraph 19

-URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4569616-111683

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Period Type:	duration

- Definition

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph h

-Article 4

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Income Tax Expense (or Benefit)

-URI http://asc.fasb.org/extlink&oid=6515339

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a),(b)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319

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- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

+ Details

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- Definition

The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585

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- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of operating expense for routine plant maintenance, repairs and operations of regulated operation.

+ References

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Interest expense, net of AFUDC which represents capitalized costs used to finance the construction of utility plants.

+ References

No definition available.

+ Details

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