Derivative Financial Instruments (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Amounts relevant to offsetting of derivative assets and liabilities
Amounts presented in consolidated balance sheets, Total derivative assets	$ 52.8	$ 74.3
Amounts presented in consolidated balance sheets, Total derivative liabilities	0.5
Amounts presented in consolidated balance sheets, Net assets	52.3	74.3
Gross amounts not offset in consolidated balance sheets
Financial instruments, Total derivative assets	0.5	0.2
Financial instruments, Total derivative liabilities	0.5
Financial instruments, Net assets (liabilities)		0.2
Net Amount, Total derivative assets	52.3	74.1
Net Amount, Net assets (liabilities)	52.3	74.1
Exposure to credit loss from nonperformance by counterparties to derivative instruments	$ 52.3	$ 74.1