Common Stock - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair market value of awards vesting	$ 14.3	$ 13.4	$ 10.2
Unrecognized compensation cost related to all non-vested awards	17.4
Aggregate intrinsic value of stock options exercised	0.3	1.5	0.7
Cash received from all option exercises	1.1	5.0	2.9
Income tax benefit realized from stock option exercises	0.1	0.6	0.3
Common equity issued for DRIP			$ 3.7
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based grants can vest	150.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based grants can vest	0.00%
2010 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reduction in number of shares of common stock subject to grants	3,000,000
Common stock subject to grant	4,000,000
Actual restricted shares granted	1,000,000	800,000	800,000
Granted, Weighted Average Grant Date Fair Value	$ 15.96	$ 18.44	$ 17.22

Percentage of performance based grants can vest

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- Definition

Reduction in number of shares of common stock subject to grants

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- Definition

Items including common stock value, paid in capital, retained earnings, common stock purchased for the treasury and excluding noncontrolling interests (minority interests) and preferred stockholder interests.

+ References

No definition available.

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- Definition

As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

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- Definition

The total cash inflow associated with the amount received from holders to acquire the entity's shares under incentive and share awards, including stock option exercises. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

+ References

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

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-Name Accounting Standards Codification

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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).

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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).

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- Definition

The total fair value of equity-based awards 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.

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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).

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- Definition

The total accumulated difference between fair values of underlying shares on dates of exercise and exercise price on options which were exercised (or share units converted) into shares during the reporting period under the plan.

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- Definition

Tax benefits realized from the exercise of stock options resulting in reductions in the entity's income taxes, which increases net cash provided by operating activities.

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