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 of the assets less the liabilities of a derivative or group of derivatives.

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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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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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The sum of the realized and unrealized net gain (loss) on trading securities during the period.

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No definition available.

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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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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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No definition available.

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

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Number of Investments

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Represents the fair value of trading debt securities re-classified to assets held for sale.

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Investment Securities (Rollforward of Fair Value Measurements by Level) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			$ 3,208,300,000	$ 2,547,300,000
Net realized and unrealized gains (losses)			31,300,000	125,600,000
Amortization/Accretion			(2,200,000)	(6,600,000)
Purchases			1,151,900,000	2,078,200,000
Sales			(2,024,200,000)	(2,229,900,000)
Transfers in			0	13,100,000
Transfers out			0	(13,100,000)
Ending balance	$ 2,365,100,000	$ 2,509,400,000	2,365,100,000	2,509,400,000
Net realized and unrealized investment (losses) gains	66,100,000	32,500,000	22,900,000	102,500,000
Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			890,400,000	279,500,000
Net realized and unrealized gains (losses)			51,900,000	52,400,000
Amortization/Accretion			100,000	0
Purchases			412,600,000	940,700,000
Sales			(457,600,000)	(539,000,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	897,400,000	733,600,000	897,400,000	733,600,000
Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			2,105,400,000	2,093,800,000
Net realized and unrealized gains (losses)			(33,600,000)	59,800,000
Amortization/Accretion			(2,300,000)	(6,600,000)
Purchases			673,500,000	1,038,500,000
Sales			(1,565,000,000)	(1,668,200,000)
Transfers in			0	13,100,000
Transfers out			0	0
Ending balance	$ 1,178,000,000	1,525,200,000	$ 1,178,000,000	1,525,200,000
Number of Investments	2		2
Fixed maturity investments \| Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			$ 0	0
Net realized and unrealized gains (losses)			0	0
Amortization/Accretion			0	0
Purchases			0	31,200,000
Sales			0	(12,500,000)
Transfers in			0	0
Transfers out			0	(13,100,000)
Ending balance	$ 0	5,600,000	0	5,600,000
Other long-term investments \| Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			87,200,000	91,400,000
Net realized and unrealized gains (losses)			(4,200,000)	(2,200,000)
Amortization/Accretion			0	0
Purchases			55,100,000	2,900,000
Sales			0	(2,000,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	138,100,000	90,100,000	138,100,000	90,100,000
Hedge and private equity funds included in other long-term investments \| Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			125,300,000	82,600,000
Net realized and unrealized gains (losses)			17,200,000	15,600,000
Amortization/Accretion			0	0
Purchases			10,700,000	64,900,000
Sales			(1,600,000)	(8,200,000)
Transfers in			0	0
Transfers out			0	0
Ending balance	151,600,000	154,900,000	151,600,000	154,900,000
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains			(700,000.0)
Foreign Exchange Forward
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains			(3,500,000.0)	(23,100,000.0)
Foreign Currency on Cash and Open Trades
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains			(4,200,000.0)
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			3,259,100,000
Ending balance	2,527,200,000		2,527,200,000
Fair value measured on a recurring basis \| Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			1,041,400,000
Ending balance	1,198,100,000		1,198,100,000
Fair value measured on a recurring basis \| Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			2,130,500,000
Ending balance	1,191,000,000		1,191,000,000
Fair value measured on a recurring basis \| Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			87,200,000
Ending balance	138,100,000		138,100,000
Fair value measured on a recurring basis \| Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			2,129,700,000
Ending balance	1,031,900,000		1,031,900,000
Fair value measured on a recurring basis \| Fixed maturity investments \| Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			296,500,000
Ending balance	125,400,000		125,400,000
Fair value measured on a recurring basis \| Fixed maturity investments \| Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			1,833,200,000
Ending balance	906,500,000		906,500,000
Fair value measured on a recurring basis \| Fixed maturity investments \| Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			0
Ending balance	0		0
Fair value measured on a recurring basis \| Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			87,200,000
Ending balance	138,100,000		138,100,000
Fair value measured on a recurring basis \| Other long-term investments \| Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			0
Ending balance	0		0
Fair value measured on a recurring basis \| Other long-term investments \| Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			0
Ending balance	0		0
Fair value measured on a recurring basis \| Other long-term investments \| Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			87,200,000
Ending balance	138,100,000		138,100,000
Fair value measured on a recurring basis \| Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			176,100,000
Short-term investments, at fair value	313,700,000		313,700,000
Fair value measured on a recurring basis \| Short-term investments \| Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			151,000,000
Ending balance	300,700,000		300,700,000
Short-term investments, at fair value		786,500,000		786,500,000	$ 176,100,000	$ 175,000,000
Fair value measured on a recurring basis \| Short-term investments \| Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			25,100,000
Ending balance	13,000,000		13,000,000
Fair value measured on a recurring basis \| Short-term investments \| Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Beginning balance			0
Ending balance	$ 0		$ 0
Forward Contracts \| Fair value measured on a recurring basis \| Other long-term investments \| Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Derivative, fair value, net		$ (15,400,000)		(15,400,000)	$ (3,700,000)	(1,200,000)
Star & Shield Insurance Exchange
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				5,200,000
Star & Shield Insurance Exchange \| Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				0
Star & Shield Insurance Exchange \| Level 2 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				5,200,000
Star & Shield Insurance Exchange \| Fixed maturity investments \| Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				0
Star & Shield Insurance Exchange \| Other long-term investments \| Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				0
Star & Shield Insurance Exchange \| Hedge and private equity funds included in other long-term investments \| Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Deconsolidation of SSIE				$ 0
Star & Shield Insurance Exchange
Debt and Equity Securities, FV-NI [Line Items]
Trading securities debt reclassified to assets held-for-sale						6,600,000
Star & Shield Insurance Exchange \| Fair value measured on a recurring basis \| Short-term investments \| Level 1 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value						$ 100,000