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Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Maturity period of cash and cash equivalents	Three months or less

X

- Definition

Maturity period of cash and cash equivalents.

+ References

No definition available.

+ Details

Name:	glt_CashAndCashEquivalentsMaturityDescription
Namespace Prefix:	glt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_AccountingPoliciesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration