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Percentage of performance based grants can vest.

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Potential additional shares to be issued.

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Proceeds from issuance of stock to underwriters.

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Reduction in number of shares of common stock subject to grants.

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shares issued upon partial exercise of underwriters option to purchase additional shares.

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Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

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Unrecognized cost of unvested share-based compensation awards.

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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.

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Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

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The cash inflow associated with the amount received from entity's raising of capital via private rather than public placement.

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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.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Per share or per unit amount of equity securities issued.

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Number of new stock issued during the period.

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Common Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Sep. 04, 2015
Class Of Stock [Line Items]
Non-vested awards compensation costs, period for recognition		2 years
Actual restricted shares granted		700,000	800,000	700,000
Granted, weighted average grant date fair value		$ 22.96	$ 14.69	$ 17.21
Total fair market value of awards vesting		$ 7.5	$ 3.6	$ 3.5
Unrecognized compensation cost related to all non-vested awards		13.2
Aggregate intrinsic value of stock options exercised		2.9	2.7	2.4
Cash received from all option exercises		9.4	10.8	6.7
Income tax benefit realized from stock option exercises		$ 1.1	$ 1.0	$ 0.8
Maximum [Member]
Class Of Stock [Line Items]
Percentage of performance based grants can vest		150.00%
Minimum [Member]
Class Of Stock [Line Items]
Percentage of performance based grants can vest		0.00%
Restricted Stock [Member]
Class Of Stock [Line Items]
Non-vested awards compensation costs, period for recognition		3 years
Director Restricted Stock [Member]
Class Of Stock [Line Items]
Time-vested restricted stock granted to directors, vesting period		1 year
2015 Performance-Based Restricted Stock Grants Vested [Member] \| Maximum [Member]
Class Of Stock [Line Items]
Percentage of performance based grants can vest		200.00%
2015 Performance-Based Restricted Stock Grants Vested [Member] \| Minimum [Member]
Class Of Stock [Line Items]
Percentage of performance based grants can vest		0.00%
2010 Plan [Member]
Class Of Stock [Line Items]
Reduction in number of shares of common stock subject to grants		3,000,000
Common stock subject to grant		4,000,000
Underwriter [Member]
Class Of Stock [Line Items]
Number of shares issued through underwriting agreement	15,500,000
Shares offered, price per share	$ 18.10
Net proceeds from offering through the underwriting	$ 271.0
Additional shares to be offered under underwriting agreement	2,300,000
Number of days option granted to underwriters	30 days
Proceeds from issuance to underwriters	$ 21.0
Underwriters exercised this option for an additional, shares	1,200,000
Emera Inc. [Member]
Class Of Stock [Line Items]
Amount of cash offered for each acquiree common stock share outstanding					$ 27.55