Derivatives (Forward Contract Collateral) (Details) (Forward Contracts, USD $) In Millions, unless otherwise specified	Mar. 31, 2013 Rating	Dec. 31, 2012
Derivative [Line Items]
Net amount of exposure after effect of collateral held	$ 79.3	$ 31.2
Uncollaterailized balance of forward currency derivatives	(0.2)	(0.1)
S and P Credit Ratings	21
A | Barclays
Derivative [Line Items]
Net amount of exposure after effect of collateral held	24.7	7.7
Uncollaterailized balance of forward currency derivatives	0.1	(0.1)
A | Deutsche Bank
Derivative [Line Items]
Net amount of exposure after effect of collateral held	13.8	11.1
Uncollaterailized balance of forward currency derivatives	(0.2)	0
A | JP Morgan
Derivative [Line Items]
Net amount of exposure after effect of collateral held	22.1	1.9
Uncollaterailized balance of forward currency derivatives	0.1	0
A- | Goldman Sachs
Derivative [Line Items]
Net amount of exposure after effect of collateral held	1.1	0.4
Uncollaterailized balance of forward currency derivatives	0	0
AA- | HSBC
Derivative [Line Items]
Net amount of exposure after effect of collateral held	0.1	10.1
Uncollaterailized balance of forward currency derivatives	0	0
AA- | Royal Bank of Canada
Derivative [Line Items]
Net amount of exposure after effect of collateral held	17.5	0
Uncollaterailized balance of forward currency derivatives	$ (0.2)	$ 0