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Investments - Narrative (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 18, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Sep. 19, 2018	Sep. 18, 2018	Jun. 30, 2018	May 10, 2018	Jan. 01, 2017	Jul. 26, 2016	Jan. 01, 2015
Investment Holdings [Line Items]
Catastrophe bonds		$ 37,400,000		$ 37,400,000		$ 32,400,000
Equity method investment, aggregate cost		66,900,000		66,900,000		66,400,000			$ 67,100,000
Restricted assets		4,526,400,000		4,526,400,000		4,638,800,000
Other investments		100,500,000		100,500,000		0
Investment funds		0		0		0
OTTI charge recognized		0	$ 100,000	0	$ 500,000
MVI
Investment Holdings [Line Items]
Equity method investment, aggregate cost		400,000		400,000		500,000			300,000				$ 800,000
Additional investment				100,000		100,000
Bene
Investment Holdings [Line Items]
Equity method investment, aggregate cost		3,500,000		3,500,000		2,900,000			3,800,000			$ 3,300,000
Additional investment						1,200,000
Equity method investment, ownership percentage												20.00%
Digital Re
Investment Holdings [Line Items]
Equity method investment, aggregate cost		500,000		500,000		500,000			500,000		$ 2,300,000
Equity method investment, ownership percentage											49.00%
Crop Re
Investment Holdings [Line Items]
Equity method investment, aggregate cost	$ 62,500,000	62,500,000		62,500,000		62,500,000			$ 62,500,000
Additional investment	$ 5,900,000
Equity method investment, ownership percentage	23.20%
AgriLogic Consulting, LLC
Investment Holdings [Line Items]
Equity method investment, aggregate cost								$ 0
Equity method investment, ownership percentage								40.00%
CGB DS | Crop Re
Investment Holdings [Line Items]
Equity method investment, counterparty ownership percentage	76.80%
Limited Partner | Real estate funds
Investment Holdings [Line Items]
Restricted assets		$ 0		$ 0		$ 100,000,000
Other investments							$ 13,800,000			$ 86,200,000
Disposed of by Sale | Consulting Relationships | AgriLogic Consulting, LLC
Investment Holdings [Line Items]
Sale of interest of consulting business								60.00%

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Catastrophe Bond

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