Commitments and contingencies (Details) (Subscription agreement [Member], Preferred stock [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 21, 2012
Subscription agreement [Member] \| Preferred stock [Member]
Other Commitments [Line Items]
Commitment to invest in non-voting preferred stock	$ 18.0	$ 18.0

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

No definition available.

Name:	us-gaap_OtherCommitment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant