Restrictions on cash and investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Cash and Cash Equivalents [Abstract]
Investments restricted for statutory reserve funds	$ 438.5stc_StatutorySurplusReserveFundBalance	$ 450.6stc_StatutorySurplusReserveFundBalance
Restricted cash and cash equivalent	$ 57.4	$ 15.1

- Definition

The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. For a classified balance sheet represents the current portion only (the noncurrent portion has a separate concept); there is a separate and distinct element for unclassified presentations.

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