Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Property) (Details) - Property - Specialty Products - 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	$ 375.0
All outstanding liabilities, net of reinsurance	0.0
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	30.2
2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	8.6	$ 8.6	$ 8.6	$ 8.7	$ 7.0	$ 7.0	$ 7.0	$ 6.8	$ 3.7	$ 1.8
2008
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	26.8	26.8	26.8	26.8	26.0	25.5	25.2	23.5	$ 15.5
2009
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	51.6	51.6	51.6	51.9	51.9	51.2	47.3	$ 41.5
2010
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	55.7	55.7	55.7	55.2	53.3	47.9	$ 43.1
2011
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	60.5	60.4	60.1	59.7	59.0	$ 51.8
2012
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	78.8	79.0	78.3	72.5	$ 58.8
2013
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	27.4	27.1	26.9	$ 24.3
2014
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	31.2	28.2	$ 19.8
2015
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	22.1	$ 15.0
2016
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 12.3