v3.3.1.900

Summary of significant accounting policies - Equity comp (Details) - Long-term incentive plan	12 Months Ended
	Dec. 31, 2015
Equity compensation plans
Portion of vested notional units expected to be redeemed in cash	33.00%
Portion of vested notional units expected to be redeemed in shares	67.00%
Weighted average forfeiture rate	11.00%

X

- Definition

Represents the portion of vested notional units that are expected to be redeemed in cash under the equity-based compensation plan.

+ References

No definition available.

+ Details

Name:	at_ShareBasedCompensationArrangementByShareBasedPaymentAwardPortionOfVestedNotionalUnitsExpectedToBeRedeemedInCash
Namespace Prefix:	at_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the portion of vested notional units that are expected to be redeemed in shares of common stock under the equity-based compensation plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	at_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Weighted Average Forfeiture Rate

+ References

No definition available.

+ Details

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Namespace Prefix:	at_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- 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.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_PlanNameAxis=at_LongTermIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: