Quarterly Financial Information - Unaudited (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Abstract]
Revenues	$ 373.2us-gaap_Revenues	$ 157.9us-gaap_Revenues	$ 201.3us-gaap_Revenues	$ 456.9us-gaap_Revenues	$ 397.9us-gaap_Revenues	$ 149.6us-gaap_Revenues	$ 195.6us-gaap_Revenues	$ 476.9us-gaap_Revenues	$ 1,189.3us-gaap_Revenues	$ 1,220.0us-gaap_Revenues	$ 1,098.9us-gaap_Revenues
Operating income (loss)	111.7us-gaap_OperatingIncomeLoss	68.9us-gaap_OperatingIncomeLoss	76.5us-gaap_OperatingIncomeLoss	148.3us-gaap_OperatingIncomeLoss	122.6us-gaap_OperatingIncomeLoss	(19.4)us-gaap_OperatingIncomeLoss	71.2us-gaap_OperatingIncomeLoss	131.4us-gaap_OperatingIncomeLoss	405.4us-gaap_OperatingIncomeLoss	305.8us-gaap_OperatingIncomeLoss	375.7us-gaap_OperatingIncomeLoss
Net income (loss)	62.5us-gaap_NetIncomeLoss	38.6us-gaap_NetIncomeLoss	40.3us-gaap_NetIncomeLoss	85.1us-gaap_NetIncomeLoss	68.1us-gaap_NetIncomeLoss	(19.2)us-gaap_NetIncomeLoss	39.4us-gaap_NetIncomeLoss	72.9us-gaap_NetIncomeLoss	226.5us-gaap_NetIncomeLoss	161.2us-gaap_NetIncomeLoss	212.0us-gaap_NetIncomeLoss
Asset impairment charge, before income taxes					$ 0us-gaap_AssetImpairmentCharges	$ 80.6us-gaap_AssetImpairmentCharges	$ 0us-gaap_AssetImpairmentCharges	$ 0us-gaap_AssetImpairmentCharges	$ 2.0us-gaap_AssetImpairmentCharges	$ 80.6us-gaap_AssetImpairmentCharges	$ 0us-gaap_AssetImpairmentCharges
Basic earnings (loss) per common share	$ 0.36us-gaap_EarningsPerShareBasic	$ 0.22us-gaap_EarningsPerShareBasic	$ 0.23us-gaap_EarningsPerShareBasic	$ 0.48us-gaap_EarningsPerShareBasic	$ 0.39us-gaap_EarningsPerShareBasic	$ (0.11)us-gaap_EarningsPerShareBasic	$ 0.22us-gaap_EarningsPerShareBasic	$ 0.42us-gaap_EarningsPerShareBasic	$ 1.29us-gaap_EarningsPerShareBasic	$ 0.92us-gaap_EarningsPerShareBasic	$ 1.20us-gaap_EarningsPerShareBasic
Diluted earnings (loss) per common share	$ 0.35us-gaap_EarningsPerShareDiluted	$ 0.22us-gaap_EarningsPerShareDiluted	$ 0.23us-gaap_EarningsPerShareDiluted	$ 0.48us-gaap_EarningsPerShareDiluted	$ 0.39us-gaap_EarningsPerShareDiluted	$ (0.11)us-gaap_EarningsPerShareDiluted	$ 0.22us-gaap_EarningsPerShareDiluted	$ 0.41us-gaap_EarningsPerShareDiluted	$ 1.29us-gaap_EarningsPerShareDiluted	$ 0.92us-gaap_EarningsPerShareDiluted	$ 1.19us-gaap_EarningsPerShareDiluted

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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