Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) $ in Millions	12 Months Ended
Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($) Investment	Dec. 31, 2014 USD ($)
Investment securities
Short-term investments, at amortized cost (which approximates fair value)	$ 287.0	$ 211.2
Fixed maturity investments, at fair value	4,250.2	2,630.2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	4,050.9	3,343.6
Total realized and unrealized gains (losses)	(11.6)	226.8
Amortization/Accretion	(19.3)	(19.6)
Purchases	6,381.7	2,377.1
Sales	(5,385.2)	(2,210.1)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		(61.4)
Transfers in	118.0	41.8
Transfers out	(118.0)	(41.8)
Fair value investments	5,039.7	4,050.9	$ 3,343.6
Net realized and unrealized investment gains	10.3	225.4	78.5
Trading Securities, Change in Unrealized Holding Gain (Loss)	(267.3)	152.9	(79.3)
Discontinued Operations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains	(1.5)	15.1	205.4
Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	4,134.4
Fair value investments	5,195.9	4,134.4
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,152.2	550.6
Total realized and unrealized gains (losses)	11.3	263.0
Amortization/Accretion	0.1	0.0
Purchases	2,243.7	814.5
Sales	(2,794.8)	(825.7)
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)
Fair value investments	612.5	1,152.2	550.6
Level 1 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,363.5
Fair value investments	887.0	1,363.5
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,531.4	2,372.9
Total realized and unrealized gains (losses)	0.2	(13.7)
Amortization/Accretion	(19.4)	(19.6)
Purchases	3,979.8	1,436.0
Sales	(2,491.5)	(1,286.0)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0.0
Transfers in	118.0	41.8
Transfers out	0.0	0.0
Fair value investments	4,118.5	2,531.4	2,372.9
Level 2 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,531.4
Fair value investments	4,131.2	2,531.4
Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Trading Securities, Change in Unrealized Holding Gain (Loss)	6.2	(23.1)	7.7
Level 3 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	239.5
Fair value investments	177.7	239.5
Prospector
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains		0.8
Symetra
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchases		394.5
Symetra \| Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchases		394.5
Trading Securities, Change in Unrealized Holding Gain (Loss)		258.8
Star & Shield Insurance Exchange [Member] \| Fixed maturity investments
Investment securities
Fixed maturity investments, at fair value	6.6	9.5	10.1
Common equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Trading Securities, Change in Unrealized Holding Gain (Loss)	(254.6)	207.6	(83.0)
Common equity securities \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0.0	39.5
Total realized and unrealized gains (losses)	0.0	7.8
Amortization/Accretion	0.0	0.0
Sales	0.0	(43.7)
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
Fair value investments	0.0	0.0	39.5
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.0	(9.0)	5.8
Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Trading Securities, Change in Unrealized Holding Gain (Loss)	(12.1)	(15.6)	11.3
Fixed maturity investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,639.7
Fair value investments	4,256.8	2,639.7
Fixed maturity investments \| Level 1 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	134.0
Fair value investments	269.4	134.0
Fixed maturity investments \| Level 2 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	2,435.7
Fair value investments	3,987.4	2,435.7
Fixed maturity investments \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	70.0	76.4
Total realized and unrealized gains (losses)	0.1	(1.1)
Amortization/Accretion	0.0	0.0
Purchases	120.8	35.3
Sales	(72.9)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0.0
Transfers in	0.0	0.0
Transfers out	(118.0)	(40.6)
Fair value investments	$ 0.0	$ 70.0	76.4
Number of investments \| Investment	3	6
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ 0.1	$ (1.1)	1.9
Fixed maturity investments \| Level 3 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	70.0
Fair value investments	0.0	70.0
Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Trading Securities, Change in Unrealized Holding Gain (Loss)	(0.6)	(39.1)	(7.6)
Other long-term investments \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	169.5
Fair value investments	177.7	169.5
Other long-term investments \| Level 1 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0.0
Fair value investments	0.0	0.0
Other long-term investments \| Level 2 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0.0
Fair value investments	0.0	0.0
Other long-term investments \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	169.5	125.9
Total realized and unrealized gains (losses)	6.1	(20.0)
Amortization/Accretion	0.0	0.0
Purchases	2.2	76.5
Sales	(0.1)	(12.9)
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
Fair value investments	177.7	169.5	125.9
Trading Securities, Change in Unrealized Holding Gain (Loss)	6.1	(13.0)	0.0
Other long-term investments \| Level 3 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	169.5
Fair value investments	177.7	169.5
Carrying value of investment accounted for using the equity method	3.5	3.8	5.2
Other long-term investments \| Affordable Housing Development Fund [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	14.7	16.8
Fair value investments	12.3	14.7	16.8
Short-term Investments [Member] \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	211.3	376.8
Fair value investments	287.1	211.3	376.8
Short-term Investments [Member] \| Level 1 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	211.3
Fair value investments	274.4	211.3
Short-term Investments [Member] \| Level 2 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0.0
Fair value investments	12.7	0.0
Short-term Investments [Member] \| Level 3 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0.0
Fair value investments	0.0	0.0
Short-term Investments [Member] \| Star & Shield Insurance Exchange [Member] \| Level 1 Inputs \| Short-term Investments [Member]
Investment securities
Short-term investments, at amortized cost (which approximates fair value)	0.1	0.1
Forward Contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.0
Forward Contracts [Member] \| Level 3 Inputs \| Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	(1.2)
Total hedge and private equity funds included in other long-term investments \| Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	127.8	178.3
Total realized and unrealized gains (losses)	(6.1)	(9.2)
Amortization/Accretion	0.0	0.0
Purchases	35.2	14.8
Sales	(25.9)	(41.8)
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
Fair value investments	$ 131.0	$ 127.8	$ 178.3

The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method.

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Fair value portion of investments accounted under the equity method.

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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This element represents the amount of the total realized and unrealized (holding) gains or losses for the period which are included in the statement of income.

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This element represents transfers in to one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis.

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This element represents transfers out of one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis.

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Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates

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Number of Investments

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