Stock-Based Compensation Expense (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation in operation and maintenance expense	$ 4,171	$ 4,420	$ 6,882	$ 6,462
Income tax benefit	(1,627)	(1,724)	(2,684)	(2,520)
After-tax stock-based compensation expense	2,544	2,696	4,198	3,942
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation in operation and maintenance expense	702	1,100	1,357	1,852
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation in operation and maintenance expense	3,321	3,177	5,240	4,334
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation in operation and maintenance expense	$ 148	$ 143	$ 285	$ 276

The amount of expense, net of income tax, recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

No definition available.

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- Definition

The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during 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 (h)(1)(i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

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

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