Condensed Consolidating Financial Statements - Additional Information (Detail) (2017 and 2018 Notes [Member])	12 Months Ended
Dec. 31, 2013
2017 and 2018 Notes [Member]
Condensed Financial Statements, Captions [Line Items]
Ownership percentage	100.00%
Dividend payment threshold of percentage of future net income	50.00%

Dividend payment threshold of percentage of future net income.

No definition available.

Name:	ck0000790528_DividendPaymentThresholdOfPercentageOfFutureNetIncome
Namespace Prefix:	ck0000790528_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration