Accounting Policies - Additional Information (Detail)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Accounting policies [line items]
Cash flow projection period		five years
Future cash flows projections growth rate	3.10%
Pre-tax discount rates	10.70%
Bottom of Range [Member]
Accounting policies [line items]
Current quantitative requirement for hedge effectiveness test	80.00%	80.00%
Net income for each period absorbed, percentage		30.00%
Top of Range [Member]
Accounting policies [line items]
Current quantitative requirement for hedge effectiveness test	125.00%	125.00%
Computer Software [Member] \| Weighted average [Member]
Accounting policies [line items]
Intangible assets, estimated useful life		4 years

Accounting policies.

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- Definition

Cash flows of hedging instrument effective rate.

+ References

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- Definition

Intangible assets estimated useful life

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Net income for each period absorbed percentage.

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- Definition

The discount rate applied to cash flow projections for a cash-generating unit (group of units). [Refer: Cash-generating units [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The growth rate used to extrapolate cash flow projections beyond the period covered by the most recent budgets/forecasts for a cash-generating unit (group of units). [Refer: Cash-generating units [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

The description of periods when cash flows are expected to occur for cash flow hedges. [Refer: Cash flow hedges [member]]

+ References

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