Notes Payable and Debt and Financing Arrangements - Notes payable and debt (Details) € in Thousands, $ in Thousands					3 Months Ended		6 Months Ended
		Jul. 07, 2017 USD ($)	Jun. 29, 2017	Jun. 15, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 EUR (€)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Gain (Loss) on Extinguishment of Debt								$ 0
Interest expense	[1]				$ 43,422	$ 43,733	$ 83,672	$ 85,346
Commitment, debt	[2]				3,243,800		3,243,800			$ 2,675,000
Notes payable and debt	[3]				3,207,339		$ 3,207,339			2,647,677
Percentage ownership of subsidiaries							100.00%
Senior notes | 2.3% Senior Notes due 2018
Debt Instrument [Line Items]
Commitment, debt	[2]				300,000		$ 300,000			300,000
Notes payable and debt	[3]				$ 299,021		$ 299,021			298,686
Debt interest rate (percent)					2.30%		2.30%		2.30%
Senior notes | 5.75% Senior Notes due 2021
Debt Instrument [Line Items]
Commitment, debt	[2]				$ 400,000		$ 400,000			400,000
Notes payable and debt	[3]				$ 398,169		$ 398,169			397,953
Debt interest rate (percent)					5.75%		5.75%		5.75%
Senior notes | 6.375% Senior Notes due 2024
Debt Instrument [Line Items]
Commitment, debt	[2]				$ 350,000		$ 350,000			350,000
Notes payable and debt	[3]				$ 349,193		$ 349,193			349,139
Debt interest rate (percent)					6.375%		6.375%		6.375%
Senior notes | 6.25% Senior Notes due 2027
Debt Instrument [Line Items]
Commitment, debt	[2]				$ 325,000		$ 325,000			325,000
Notes payable and debt	[3]				$ 323,453		$ 323,453			323,375
Debt interest rate (percent)					6.25%		6.25%		6.25%
Senior notes | 5.25% Senior Notes due 2043
Debt Instrument [Line Items]
Commitment, debt	[2]				$ 300,000		$ 300,000			300,000
Notes payable and debt	[3]				$ 296,493		$ 296,493			296,427
Debt interest rate (percent)					5.25%		5.25%		5.25%
Subordinated notes | 4.45% Subordinated Notes due 2025
Debt Instrument [Line Items]
Commitment, debt	[2]				$ 500,000		$ 500,000			500,000
Notes payable and debt	[3]				$ 493,733		$ 493,733			493,329
Debt interest rate (percent)					4.45%		4.45%		4.45%
Subordinated notes | 5.5% Subordinated Notes due 2045
Debt Instrument [Line Items]
Commitment, debt	[2]				$ 500,000		$ 500,000			500,000
Notes payable and debt	[3]				$ 488,966		$ 488,966			488,768
Debt interest rate (percent)					5.50%		5.50%		5.50%
Subordinated notes | 3.25% Subordinated Notes due 2047
Debt Instrument [Line Items]
Commitment, debt	[2]				$ 568,800		$ 568,800		€ 500,000	0
Notes payable and debt	[3]				558,311		558,311			$ 0
Unamortized Debt Issuance Expense					$ 10,500		$ 10,500
Debt interest rate (percent)					3.25%		3.25%		3.25%
Notes Payable, Interest Rate Term			10 years
Debt Issuance Percentage of Face Value					99.054%		99.054%		99.054%
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%
Subsequent Event [Member]
Debt Instrument [Line Items]
Debt Instrument, Repurchased Face Amount		$ 16,300
Debt Instrument, Repurchase Amount		17,900
Gain (Loss) on Extinguishment of Debt		(1,600)
Interest expense		$ 300

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

[2] Excluded from the table are certain credit facilities under which the Company is permitted to utilize up to $750.0 million at June 30, 2017 and $1.0 billion at December 31, 2016, for revolving loans to support general operating and financing needs. At June 30, 2017 and December 31, 2016, $44.7 million and $245 million, respectively, were utilized under these facilities to issue letters of credit, leaving $705.3 million and $755.0 million, respectively, available to support other operating and financing needs.

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