Rollforward of Fair Value Measurements by Level (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			$ 4,050.9	$ 3,343.6
Total realized and unrealized gains (losses)			84.0	(32.7)
Amortization/Accretion			(13.2)	(15.2)
Purchases			4,722.9	1,672.0
Sales			(3,944.4)	(1,540.9)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				61.4
Transfers in			82.0	41.7
Transfers out			(82.0)	(41.7)
Balance at September 30, 2016	$ 4,900.2	$ 3,365.4	4,900.2	3,365.4
Other Investments	351.4		351.4		$ 315.8
Net realized and unrealized investment gains (losses), after tax	21.0	(34.5)	60.2	(27.0)
Net realized and unrealized investment gains (losses)	26.4	(43.9)	84.0	(33.9)
Short-term investments, at amortized cost (which approximates fair value)	305.6		$ 305.6		211.2
Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers			(5.00%)
Maximum price difference provided by pricing services			$ 1.0
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			1,152.2	550.6
Total realized and unrealized gains (losses)			20.0	(19.5)
Amortization/Accretion			0.0	0.0
Purchases			1,577.5	550.0
Sales			(2,337.3)	(618.2)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				43.5
Transfers in			0.0	0.0
Transfers out			0.0	(1.2)
Balance at September 30, 2016	412.4	418.2	412.4	418.2
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			2,531.4	2,372.9
Total realized and unrealized gains (losses)			42.7	0.6
Amortization/Accretion			(13.2)	(15.2)
Purchases			2,989.4	1,056.0
Sales			(1,587.5)	(894.4)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				0.0
Transfers in			82.0	41.7
Transfers out			0.0	0.0
Balance at September 30, 2016	4,044.8	2,561.6	4,044.8	2,561.6
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			70.0	76.4
Total realized and unrealized gains (losses)			1.6	(0.9)
Amortization/Accretion			0.0	0.0
Purchases			120.8	35.3
Sales			(2.8)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				0.0
Transfers in			0.0	0.0
Transfers out			(82.0)	(40.5)
Balance at September 30, 2016	107.6	70.3	107.6	70.3
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			0.0	39.5
Total realized and unrealized gains (losses)			0.0	7.7
Amortization/Accretion			0.0	0.0
Purchases			0.0	0.0
Sales			0.0	(9.4)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				3.6
Transfers in			0.0	0.0
Transfers out			0.0	0.0
Balance at September 30, 2016	0.0	34.2	0.0	34.2
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			169.5	126.9
Total realized and unrealized gains (losses)			20.7	(13.5)
Amortization/Accretion			0.0	0.0
Purchases			2.2	22.6
Sales			0.0	(2.3)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				0.0
Transfers in			0.0	0.0
Transfers out			0.0	0.0
Balance at September 30, 2016	192.4	133.7	192.4	133.7
Hedge and private equity funds included in other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			127.8	177.3
Total realized and unrealized gains (losses)			(1.0)	(7.1)
Amortization/Accretion			0.0	0.0
Purchases			33.0	8.1
Sales			(16.8)	(16.6)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates				(14.3)
Transfers in			0.0	0.0
Transfers out			0.0	0.0
Balance at September 30, 2016	143.0	147.4	143.0	147.4
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			4,134.4
Balance at September 30, 2016	5,062.8		5,062.8
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			1,363.5
Balance at September 30, 2016	691.0		691.0
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			2,531.4
Balance at September 30, 2016	4,071.8		4,071.8
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			239.5
Balance at September 30, 2016	300.0		300.0
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			2,639.7
Balance at September 30, 2016	4,139.8		4,139.8
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			134.0
Balance at September 30, 2016	88.1		88.1
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			2,435.7
Balance at September 30, 2016	3,944.1		3,944.1
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			70.0
Balance at September 30, 2016	107.6		107.6
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			169.5
Balance at September 30, 2016	192.4		192.4
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			0.0
Balance at September 30, 2016	0.0		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]
Balance at January 1, 2016			0.0
Balance at September 30, 2016	0.0		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]
Balance at January 1, 2016			169.5
Balance at September 30, 2016	192.4		192.4
Carrying value of investment accounted for using the equity method	3.2	3.9	3.2	3.9	3.8	$ 5.2
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			211.3
Balance at September 30, 2016	305.6		305.6
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			211.3
Balance at September 30, 2016	278.6		278.6
Short-term investments, at amortized cost (which approximates fair value)		325.0		325.0	211.3	376.8
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			0.0
Balance at September 30, 2016	27.0		27.0
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2016			0.0
Balance at September 30, 2016	0.0		0.0
Discontinued Operations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains (losses)	0.0	(0.9)	(1.5)	31.4
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	0.6	0.1	0.6	0.1	1.9
Prospector Offshore Fund Ltd [Member] | Fair value measured on a recurring basis | Investment related liabilities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains (losses)				0.8
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	12.8	15.2	12.8	15.2	14.7	16.8
Affordable Housing Development Fund [Member] | Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	12.8		12.8		14.7
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	$ 9.7	$ 8.3	$ 9.7	$ 9.5	$ 10.1