Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) Investment	Sep. 30, 2016 USD ($) Investment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	$ 2,547.3	$ 1,543.5
Net realized and unrealized gains (losses)	(125.6)	27.2
Amortization/Accretion	(6.6)	(3.7)
Purchases	2,078.2	3,726.9
Sales	2,229.9	2,887.0
Transfers in	13.1	68.0
Transfers out	(13.1)	(68.0)
Ending balance	2,509.4	2,406.9
Short-term investments, at amortized cost (which approximates fair value)	786.5		$ 174.9
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	279.5	789.0
Net realized and unrealized gains (losses)	52.4	7.2
Amortization/Accretion	0.0	0.1
Purchases	940.7	1,387.8
Sales	539.0	1,992.7
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	733.6	191.4
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,093.8	585.6
Net realized and unrealized gains (losses)	59.8	17.4
Amortization/Accretion	(6.6)	(3.8)
Purchases	1,038.5	2,228.5
Sales	1,668.2	884.1
Transfers in	13.1	68.0
Transfers out	0.0	0.0
Ending balance	1,525.2	2,011.6
Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0.0	0.0
Net realized and unrealized gains (losses)	0.0	0.1
Amortization/Accretion	0.0	0.0
Purchases	31.2	70.0
Sales	12.5
Transfers in	0.0	0.0
Transfers out	13.1	(68.0)
Ending balance	$ 5.6	$ 2.1
Number of investments | Investment	2	2
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	$ 91.4	$ 103.6
Net realized and unrealized gains (losses)	(2.2)	0.8
Amortization/Accretion	0.0	0.0
Purchases	2.9	2.2
Sales	2.0	0.1
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	90.1	106.5
Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	82.6	65.3
Net realized and unrealized gains (losses)	15.6	1.7
Amortization/Accretion	0.0	0.0
Purchases	64.9	38.4
Sales	8.2	10.1
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	154.9	95.3
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,639.7
Ending balance	3,141.0
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	441.7
Ending balance	1,498.1
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,106.6
Ending balance	1,547.2
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	91.4
Ending balance	95.7
Fair value measured on a recurring basis | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,087.7
Ending balance	1,490.0
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	101.5
Ending balance	217.8
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	1,986.2
Ending balance	1,266.6
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0.0
Ending balance	5.6
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	91.4
Ending balance	90.1
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0.0
Ending balance	0.0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0.0
Ending balance	0.0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	91.4
Ending balance	90.1
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	175.0
Ending balance	786.5
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	162.3
Ending balance	764.5
Short-term investments, at amortized cost (which approximates fair value)		230.0	175.0	$ 142.0
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	12.7
Ending balance	22.0
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	0.0
Ending balance	0.0
Star & Shield Insurance Exchange | Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Short-term investments, at amortized cost (which approximates fair value)		0.1	0.1	0.1
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities debt reclassified to assets held-for-sale		$ 8.3	6.6	$ 9.5
Forward Contracts | Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative, fair value, net	(15.4)		$ (1.2)
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	5.2
Star & Shield Insurance Exchange | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	0.0
Star & Shield Insurance Exchange | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	5.2
Star & Shield Insurance Exchange | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	6.6
Star & Shield Insurance Exchange | Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	0.0
Star & Shield Insurance Exchange | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	0.0
Star & Shield Insurance Exchange | Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Sales	0.0
Star & Shield Insurance Exchange | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	$ 0.1