Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Property) (Details) - Property - 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	$ 1,171.4
All outstanding liabilities, net of reinsurance	0.0
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance	108.5
2007
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	94.6	$ 94.5	$ 94.5	$ 94.2	$ 93.4	$ 91.7	$ 87.6	$ 82.7	$ 70.9	$ 42.2
2008
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	129.3	129.3	129.2	129.1	129.5	123.4	116.8	101.4	$ 62.5
2009
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	118.4	118.0	117.2	115.5	111.0	105.4	85.9	$ 50.4
2010
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	130.8	129.3	125.9	122.8	118.2	106.2	$ 69.0
2011
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	141.4	139.5	138.0	130.7	117.4	$ 72.5
2012
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	131.9	128.5	116.4	103.1	$ 64.2
2013
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	137.7	126.1	101.5	$ 59.8
2014
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	111.6	94.6	$ 56.5
2015
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	114.4	$ 68.6
2016
Claims Development [Line Items]
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance	$ 61.3