Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Abstract]
Loss, LAE and unearned premium reserves, net	$ 179.0	$ 175.9
Tax credit carryforwards	70.5	125.8
Employee benefit plans	39.6	45.7
Investments, net	12.4
Operating loss carryforwards		29.1
Other	53.6	43.8
Deferred tax assets, gross	355.1	420.3
Deferred tax assets, total	355.1	420.3
Deferred acquisition costs	134.9	129.6
Software capitalization	26.9	27.3
Deferred Lloyd's underwriting income	22.4	54.0
Investments, net		44.4
Other	33.0	33.8
Deferred tax liabilities, total	217.2	289.1
Net deferred tax asset	$ 137.9	$ 131.2