Rollforward of Fair Value Measurements by Level (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	$ 2,547.3	$ 1,543.5
Net realized and unrealized gains (losses)	(83.0)	16.3
Amortization/Accretion	(4.7)	(2.0)
Purchases	1,516.4	3,066.7
Sales	1,503.0	2,019.9
Transfers in	11.1	0.0
Transfers out	(11.1)	0.0
Ending balance	2,633.8	2,604.6
Short-term investments, at amortized cost (which approximates fair value)	71.6		$ 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)	30.4	3.6
Amortization/Accretion	0.0	0.1
Purchases	665.0	1,312.9
Sales	243.9	1,809.9
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	731.0	295.7
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)	39.0	13.7
Amortization/Accretion	(4.7)	(2.1)
Purchases	770.7	1,668.7
Sales	1,255.0	207.0
Transfers in	11.1	0.0
Transfers out	0.0	0.0
Ending balance	1,649.7	2,058.9
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)	0.1	0.1
Amortization/Accretion	0.0	0.0
Purchases	25.6	67.9
Sales	0.5
Transfers in	0.0	0.0
Transfers out	11.1	0.0
Ending balance	14.1	68.0
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)	(1.5)	1.6
Amortization/Accretion	0.0	0.0
Purchases	2.6	2.0
Sales	2.0
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	90.5	107.2
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)	15.0	(2.7)
Amortization/Accretion	0.0	0.0
Purchases	52.5	15.2
Sales	1.6	3.0
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	148.5	74.8
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Beginning balance	2,639.7
Ending balance	2,556.9
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	786.7
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	1,665.6
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	104.6
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	1,566.9
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	49.7
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	1,503.1
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	14.1
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	90.5
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	90.5
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	71.6
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	55.7
Short-term investments, at amortized cost (which approximates fair value)		307.3	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	15.9
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)		0.3	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		$ 9.2	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	(12.5)		$ (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