Notes Payable and Debt and Financing Arrangements - Narrative relating to credit facilities (Details)		Dec. 31, 2016 USD ($) Credit_Facilities	Dec. 31, 2015 USD ($) Credit_Facilities	Dec. 30, 2014 USD ($)
Line of Credit Facility [Line Items]
Notes payable and debt		$ 2,647,677,000	$ 2,644,970,000
Number of letter of credit facilities available | Credit_Facilities		15	15
Credit Agricole Facility II [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		$ 125,000,000
Funds at Lloyds Bilateral Facility [Member]
Line of Credit Facility [Line Items]
Number of letter of credit facilities available | Credit_Facilities		4
Available letter of credit facilities - commitments		$ 500,000,000
Funds at Lloyds Bilateral Facility I [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		125,000,000
Funds at Lloyds Bilateral Facility II [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		125,000,000
Funds at Lloyds Bilateral Facility III [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		125,000,000
Funds at Lloyds Bilateral Facility IV [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		125,000,000
Two Thousand Thirteen Citi Agreements [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		250,000,000
Revolving credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use		245,000,000	$ 527,100,000
Available letter of credit facilities - commitments		1,000,000,000	1,400,000,000
Line of credit facility, remaining borrowing capacity		755,000,000	822,900,000
Syndicated Credit Agreements [Member]
Line of Credit Facility [Line Items]
Optional increase capacity		500,000,000
Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	[1]	2,345,293,000	2,515,653,000
Available letter of credit facilities - commitments	[1],[2]	3,991,687,000	4,463,041,000
Credit Agricole Facility I [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		125,000,000
Twenty Fifteen Goldman Sachs Facility [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		200,000,000
Catlin Bermuda [Member] | Bilateral Facility [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use		180,000,000
Catlin Re Switzerland [Member] | Bilateral Facility [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use		250,000,000
Catlin Group Limited [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use			$ 4
Citibank Europe plc [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use		600,000,000		$ 750,000,000
Lloyds Syndicate | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use		12,000,000
Citibank N.A. [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use		250,000,000
Trust Preferred Securities Subject to Mandatory Redemption [Member] | Trust Preferred Securities Eight Million Face Value [Member] | Allied International Holdings, Inc.
Line of Credit Facility [Line Items]
Notes payable and debt		8,200,000
Two Thousand Sixteen Secured Credit Agreement [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		750,000,000
Two Thousand Sixteen Unsecured Credit Agreement [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		$ 750,000,000

[1] As of December 31, 2016 and December 31, 2015, there were fifteen available credit facilities.

[2] As of December 31, 2016 and December 31, 2015 the stated portion of allowable credit facilities permitted to be utilized for revolving loans was $1.0 billion and $1.4 billion, respectively. However, as of December 31, 2016 and December 31, 2015, $245.0 million and $527.1 million, respectively, of such facilities' limits were utilized to issue letters of credit, leaving $755.0 million and $822.9 million, respectively, available either to issue additional letters of credit or to support other operating or financing needs under these particular facilities.