Parent Holding Company Condensed Financial Information (Condensed Balance Sheets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items]
Cash and cash equivalents	$ 251.1	$ 379.3	$ 160.7	$ 138.4
Other assets	129.4us-gaap_OtherAssets	127.9us-gaap_OtherAssets
Total assets	22,729.9us-gaap_Assets	21,393.3us-gaap_Assets	19,791.3us-gaap_Assets
Short-term debt	1.1us-gaap_DebtCurrent	1.5us-gaap_DebtCurrent
Long-term debt	503.9	551.9
Other liabilities	440.1us-gaap_OtherLiabilities	330.7us-gaap_OtherLiabilities
Total liabilities	20,558.1us-gaap_Liabilities	19,240.5us-gaap_Liabilities
Total shareholders' equity	2,171.8us-gaap_StockholdersEquity	2,152.8us-gaap_StockholdersEquity	2,169.0us-gaap_StockholdersEquity	1,990.4us-gaap_StockholdersEquity
Total liabilities and shareholders' equity	22,729.9	21,393.3
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Cash and cash equivalents	70.5	125.2	39.7	34.5
Investment in subsidiaries	2,317.9	2,252.9
Receivable from subsidiaries	0.9	1.5
Notes Receivable, Affiliates	250.0	250.0
Other assets	77.9	80.3
Total assets	2,717.2	2,709.9
Long-term debt	502.9	550.0
Other liabilities	42.5	7.1
Total liabilities	545.4	557.1
Total shareholders' equity	2,171.8	2,152.8
Total liabilities and shareholders' equity	$ 2,717.2	$ 2,709.9

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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