Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Casualty) (Details) - Casualty - Specialty Industries - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 558.3
All outstanding liabilities, net of reinsurance	0.1
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	360.0
2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	35.9	$ 34.3	$ 32.9	$ 32.4	$ 29.9	$ 27.9	$ 13.8	$ 7.9	$ 5.5	$ 2.8
2008
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	20.9	20.8	21.2	19.9	17.3	14.2	11.5	6.9	$ 3.2
2009
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	47.4	46.3	46.3	42.9	34.5	25.0	16.6	$ 5.1
2010
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	68.9	65.3	57.5	51.4	37.3	21.1	$ 8.7
2011
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	96.4	88.9	69.8	53.7	34.2	$ 11.1
2012
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	79.7	67.8	50.6	31.9	$ 12.9
2013
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	96.6	72.7	45.5	$ 18.6
2014
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	61.4	40.2	$ 16.6
2015
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	39.3	$ 13.6
2016
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 11.8