Notes Payable and Debt and Financing Arrangements - Notes payable and debt (Details) $ in Thousands, € in Millions					3 Months Ended				9 Months Ended
		Jul. 07, 2017 USD ($)	Jun. 29, 2017	Jun. 15, 2016 USD ($)	Sep. 30, 2017 USD ($)		Sep. 30, 2016 USD ($)		Sep. 30, 2017 USD ($)		Sep. 30, 2016 USD ($)		Jun. 30, 2017 USD ($)	Jun. 30, 2017 EUR (€)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Commitment, debt	[1]				$ 3,245,455				$ 3,245,455						$ 2,675,000
Notes payable and debt	[2]				3,210,063				3,210,063						2,647,677
Debt Instrument, Repurchased Face Amount		$ 16,300
Debt Instrument, Repurchase Amount		17,900
Gain (Loss) on Extinguishment of Debt		(1,582)			(1,582)		$ 0		(1,582)		$ 0
Interest expense		$ 300			39,509	[3]	$ 43,025	[3]	$ 123,181	[3]	$ 128,371	[3]
Percentage ownership of subsidiaries									100.00%
Senior notes | 2.3% Senior Notes due 2018
Debt Instrument [Line Items]
Commitment, debt	[1]				300,000				$ 300,000						300,000
Notes payable and debt	[2]				$ 299,189				$ 299,189						298,686
Debt interest rate (percent)					2.30%				2.30%
Senior notes | 5.75% Senior Notes due 2021
Debt Instrument [Line Items]
Commitment, debt	[1]				$ 400,000				$ 400,000						400,000
Notes payable and debt	[2]				$ 398,277				$ 398,277						397,953
Debt interest rate (percent)					5.75%				5.75%
Senior notes | 6.375% Senior Notes due 2024
Debt Instrument [Line Items]
Commitment, debt	[1]				$ 350,000				$ 350,000						350,000
Notes payable and debt	[2]				$ 349,221				$ 349,221						349,139
Debt interest rate (percent)					6.375%				6.375%
Senior notes | 6.25% Senior Notes due 2027
Debt Instrument [Line Items]
Commitment, debt	[1]				$ 325,000				$ 325,000						325,000
Notes payable and debt	[2]				$ 323,492				$ 323,492						323,375
Debt interest rate (percent)					6.25%				6.25%
Senior notes | 5.25% Senior Notes due 2043
Debt Instrument [Line Items]
Commitment, debt	[1]				$ 300,000				$ 300,000						300,000
Notes payable and debt	[2]				$ 296,526				$ 296,526						296,427
Debt interest rate (percent)					5.25%				5.25%
Subordinated notes | 4.45% Subordinated Notes due 2025
Debt Instrument [Line Items]
Commitment, debt	[1]				$ 500,000				$ 500,000						500,000
Notes payable and debt	[2]				$ 493,936				$ 493,936						493,329
Debt interest rate (percent)					4.45%				4.45%
Subordinated notes | 5.5% Subordinated Notes due 2045
Debt Instrument [Line Items]
Commitment, debt	[1]				$ 483,305				$ 483,305						500,000
Notes payable and debt	[2]				$ 472,736				$ 472,736						488,768
Debt interest rate (percent)					5.50%				5.50%
Subordinated notes | 3.25% Subordinated Notes due 2047
Debt Instrument [Line Items]
Commitment, debt	[1]				$ 587,150				$ 587,150				$ 568,800	€ 500	0
Notes payable and debt					$ 576,686	[2]			$ 576,686	[2]			$ 558,300		$ 0	[2]
Debt interest rate (percent)					3.25%				3.25%				3.25%	3.25%
Notes Payable, Interest Rate Term			10 years
Debt Issuance Percentage of Face Value													99.054%	99.054%
Unamortized Debt Issuance Expense													$ 10,500
Subordinated notes | 3.25% Subordinated Notes due 2047 | 3 month Euribor
Debt Instrument [Line Items]
Spread on variable rate			2.90%
Allied International Holdings, Inc. | Trust Preferred Securities Subject to Mandatory Redemption [Member] | Trust Preferred Securities, Face $8 million, due 2035
Debt Instrument [Line Items]
Notes payable and debt				$ 8,200
Allied International Holdings, Inc. | Trust Preferred Securities Subject to Mandatory Redemption [Member] | Trust Preferred Securities, Face $8 million, due 2035 | 3 month LIBOR
Debt Instrument [Line Items]
Spread on variable rate				3.75%					3.75%

[1] Excluded from the table are certain credit facilities under which the Company was permitted to utilize up to $750.0 million at September 30, 2017 and $1.0 billion at December 31, 2016, for revolving loans to support general operating and financing needs. At September 30, 2017 and December 31, 2016, $2 million and $245 million, respectively, were utilized under these facilities to issue letters of credit, leaving $748.0 million and $755.0 million, respectively, available to support other operating and financing needs.

[2] "In Use/Outstanding" data represent September 30, 2017 and December 31, 2016 accreted values.

[3] Interest expense excludes interest expense related to structured products recorded in the Insurance and Reinsurance segments.