Share Based Compensation - Equity-Settled Awards (Detail) (Equity-settled awards [Member], USD $)	12 Months Ended
Dec. 31, 2012
Equity-settled awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance at beginning of year	159,000
Granted, Equivalent Units	368,000
Vested, Equivalent Units	(118,000)
Forfeited, Equivalent Units	0
Balance at end of period	409,000
Non-vested at beginning of year, Weighted Average Grant Date Fair Value	$ 20.74
Granted, Weighted Average Grant Date Fair Value	$ 17.01
Vested, Weighted Average Grant Date Fair Value	$ 15.17
Forfeited, Weighted Average Grant Date Fair Value	$ 0.00
Non-vested at end of period, Weighted Average Grant Date Fair Value	$ 18.99

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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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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- Definition

The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan 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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