Stock-Based Compensation - Schedule of Information Related to Outstanding SOSARS (Detail) (Stock Only Stock Appreciation Rights (SOSARs) [Member], USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Stock Only Stock Appreciation Rights (SOSARs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Outstanding	1,864,707	1,977,133
Granted	406,142	275,529
Exercised	(58,343)	(15,974)
Canceled / forfeited	0	0
Ending Balance, Outstanding	2,212,506	2,236,688
Beginning Balance, Weighted Average Exercise Price, Outstanding	$ 16.20	$ 13.91
Weighted Average Exercise Price, Granted	$ 24.94	$ 29.89
Weighted Average Exercise Price, Exercised	$ 13.52	$ 15.48
Weighted Average Exercise Price, Canceled / forfeited	$ 0	$ 0
Ending Balance, Weighted Average Exercise Price, Outstanding	$ 17.88	$ 15.86
Weighted average grant date fair value per share	$ 7.54	$ 9.85
Aggregate grant date fair value	$ 3,063	$ 2,713
Black-Scholes assumptions
Dividend yield	1.92%	1.47%
Risk free rate of return	1.64%	1.73%
Volatility	36.48%	37.59%
Expected life	6 years	6 years

Stock-Based Compensation - Schedule of Information Related to Outstanding SOSARS (Detail) (Stock Only Stock Appreciation Rights (SOSARs) [Member], USD $)
In Thousands, except Share data, unless otherwise specified

3 Months Ended

Mar. 31, 2015

Mar. 31, 2014

Stock Only Stock Appreciation Rights (SOSARs) [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Beginning Balance, Outstanding

1,864,707us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

1,977,133us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
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= us-gaap_StockAppreciationRightsSARSMember

Granted

406,142us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

275,529us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

Exercised

(58,343)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

(15,974)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

Canceled / forfeited

0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod
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= us-gaap_StockAppreciationRightsSARSMember

Ending Balance, Outstanding

2,212,506us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

2,236,688us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
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= us-gaap_StockAppreciationRightsSARSMember

Beginning Balance, Weighted Average Exercise Price, Outstanding

$ 16.20us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

$ 13.91us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
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= us-gaap_StockAppreciationRightsSARSMember

Weighted Average Exercise Price, Granted

$ 24.94us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
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= us-gaap_StockAppreciationRightsSARSMember

$ 29.89us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

Weighted Average Exercise Price, Exercised

$ 13.52us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

$ 15.48us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

Weighted Average Exercise Price, Canceled / forfeited

$ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice
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= us-gaap_StockAppreciationRightsSARSMember

Ending Balance, Weighted Average Exercise Price, Outstanding

$ 17.88us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

$ 15.86us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

Weighted average grant date fair value per share

$ 7.54glt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantDateFairValuePerShare
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

$ 9.85glt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantDateFairValuePerShare
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

Aggregate grant date fair value

$ 3,063glt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

$ 2,713glt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

Black-Scholes assumptions

Dividend yield

1.92%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

1.47%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

Risk free rate of return

1.64%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

1.73%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

Volatility

36.48%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

37.59%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_StockAppreciationRightsSARSMember

Expected life

6 years

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Weighted average expected volatility rate of share-based compensation awards.

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- Definition

For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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- Definition

Weighted average price of options that were either forfeited or expired.

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Gross number of share options (or share units) granted during the period.

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- Definition

Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Number of share options (or share units) exercised during the current period.

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