Amounts Reclassified Out of Accumulated Other Comprehensive Income - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Amounts reclassified out of AOCI pre-tax basis	$ 0.5	$ 0.1	$ 0.5	$ 0.3

Amount before tax of reclassification adjustments of other comprehensive income (loss).

No definition available.

Name:	us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration