Summary of Significant Accounting Policies - Net Income (Loss) Per Unit (Details) - $ / shares	3 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Dilutive effect of theoretical distributions of net income on earnings (in usd per share)	$ 0.02	$ 0

Dilutive effect of theoretical distributions of net income on earnings.

No definition available.

Name:	apu_DilutiveEffectOfTheoreticalDistributionsOfNetIncomeOnEarnings
Namespace Prefix:	apu_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration