Unrealized and realized gains and losses (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reported in comprehensive income
Unrealized gains	$ 0.3	$ 0.5	$ 0.9
Unrealized losses	0.4	0	0.5
Reported in other income, net
Realized gains	0	0.2	1.0
Realized losses	$ 0.3	$ 0	$ 0.6

The increase (decrease), resulting in a gain, in the difference between the fair value and the carrying value, or in the comparative fair values, of securities held at each balance sheet date, that was included in earnings for the period.

No definition available.

Name:	us-gaap_UnrealizedGainOnSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration