Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Other) (Details) - Other - Specialty Industries $ in Millions	Dec. 31, 2016 USD ($) reported_claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Claims Development [Line Items]
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 294.8
2007
Claims Development [Line Items]
Cumulative number of reported claims | reported_claim	291
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 4.4	$ 4.4	$ 4.4	$ 4.4	$ 4.4	$ 4.4	$ 4.4	$ 4.5	$ 4.6	$ 4.3
Total IBNR plus expected development on reported claims	$ 0.0
2008
Claims Development [Line Items]
Cumulative number of reported claims | reported_claim	975
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 12.3	12.1	12.1	11.8	11.9	11.9	12.2	10.8	$ 10.4
Total IBNR plus expected development on reported claims	$ 0.0
2009
Claims Development [Line Items]
Cumulative number of reported claims | reported_claim	1,763
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 21.7	21.5	21.4	20.9	19.3	19.0	19.2	$ 19.1
Total IBNR plus expected development on reported claims	$ 0.2
2010
Claims Development [Line Items]
Cumulative number of reported claims | reported_claim	2,449
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 28.4	28.4	28.2	27.9	27.1	26.6	$ 25.6
Total IBNR plus expected development on reported claims	$ 0.0
2011
Claims Development [Line Items]
Cumulative number of reported claims | reported_claim	3,638
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 38.2	38.2	38.5	37.7	36.0	$ 35.1
Total IBNR plus expected development on reported claims	$ 0.2
2012
Claims Development [Line Items]
Cumulative number of reported claims | reported_claim	3,861
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 41.6	41.1	40.1	39.6	$ 42.0
Total IBNR plus expected development on reported claims	$ 0.3
2013
Claims Development [Line Items]
Cumulative number of reported claims | reported_claim	4,245
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 42.5	42.2	41.2	$ 41.1
Total IBNR plus expected development on reported claims	$ 0.7
2014
Claims Development [Line Items]
Cumulative number of reported claims | reported_claim	3,497
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 40.6	40.4	$ 42.6
Total IBNR plus expected development on reported claims	$ 1.9
2015
Claims Development [Line Items]
Cumulative number of reported claims | reported_claim	3,301
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 31.9	$ 46.6
Total IBNR plus expected development on reported claims	$ 4.9
2016
Claims Development [Line Items]
Cumulative number of reported claims | reported_claim	2,222
Incurred Loss and Allocated LAE, Net of Reinsurance	$ 33.2
Total IBNR plus expected development on reported claims	$ 19.4