Share-Based Compensation - Additional Information (Detail) - $ / shares	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015
Performance Shares \| Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in the period	411,178	195,814
Shares granted at maximum performance level	822,356	440,582
Weighted average share price (USD per share)	$ 19.33	$ 32.29
Performance period	3 years	3 years
Percent of the target grant amount	100.00%	100.00%
Decrease in plan target if TSR threshold is not met		25.00%
Increase in plan target if TSR threshold is met		25.00%
Performance Shares \| Employees \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of the target grant amount	0.00%	0.00%
Performance Shares \| Employees \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of the target grant amount	200.00%	225.00%
Performance Shares \| Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in the period	81,803
Shares granted at maximum performance level	163,606
Weighted average share price (USD per share)	$ 17.97
Percent of the target grant amount	100.00%
Restricted Stock Units \| Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in the period	506,493	127,899
Weighted average share price (USD per share)	$ 18.70	$ 32.82
Vesting period	3 years	3 years
Restricted Stock Units \| Employees \| Period two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vested in period (in shares)	56,524
Shares vested in period (in percentage)	25.00%	25.00%
Restricted Stock Units \| Employees \| Period three
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vested in period (in shares)	175,852
Shares vested in period (in percentage)	50.00%	50.00%
Restricted Stock Units \| Employees \| Over period three
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vested in period (in shares)	274,117
Restricted Stock Units \| Employees \| Period one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vested in period (in percentage)	25.00%	25.00%
Restricted Stock Units \| Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in the period	104,619
Weighted average share price (USD per share)	$ 17.97
Vesting period	3 years
Restricted Stock Units \| Chief Executive Officer \| Period two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vested in period (in shares)	50,083
Restricted Stock Units \| Chief Executive Officer \| Over period three
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares vested in period (in shares)	54,536
Restricted Stock Units \| Non-management members of the Board of Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in the period	25,704	24,703
Weighted average share price (USD per share)	$ 23.34	$ 28.34
Vesting period	1 year	1 year
Performance Retention Award \| Executives
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted in the period		212,963
Shares granted at maximum performance level		425,926
Weighted average share price (USD per share)		$ 32.82
Performance period		3 years
Percent of the target grant amount		100.00%
Performance Retention Award \| Executives \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of the target grant amount		50.00%
Performance Retention Award \| Executives \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of the target grant amount		200.00%

Share based compensation arrangement by share based payment award percentage of shares vested at time of delivery.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Grants in Period, Potential Vested Shares at Maximum Performance Level.

+ References

No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Plan Modification, Increase in Targets if Conditions Met.

+ References

No definition available.

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Plan Modification, Reduction in Targets if Conditions Not Met.

+ References

No definition available.

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- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ 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 (a)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

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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- 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).

+ 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 (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

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).

+ 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 (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting 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 (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Percentage of vesting of share-based compensation awards.

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