Credit Facilities (Narrative) (Details) £ in Thousands, $ in Thousands			12 Months Ended
	Jun. 22, 2012 USD ($)	Jul. 27, 2007 USD ($)	Dec. 31, 2015 GBP (£) item	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Nov. 09, 2015 GBP (£)
Line of Credit Facility [Line Items]
Line of credit facility, number of facilities | item			2
Line of credit facility, commitment					$ 1,100,000
Wells Fargo Bank Group Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, term of credit facility	4 years	5 years
Line of credit facility, commitment	$ 800,000	$ 850,000		$ 800,000	$ 800,000
Line of credit facility, required minimum debt-to-capital ratio			0.35		0.35
Line of credit facility, required minimum net worth					$ 4,249,963
Percentage amount of consolidated net income required to be reserved to maintain compliance with facility covenants			25.00%
Percentage amount of increase in consolidated net worth from new issuance required to be reserved to maintain compliance with facility covenants			25.00%
Increase in reserves due to new issuance during the period required to maintain compliance with facility covenants					5,368,970
Wells Fargo Bank Group Credit Facility [Member] | Federal Funds [Member]
Line of Credit Facility [Line Items]
Line of credit facility, spread on variable rate basis			0.50%
Wells Fargo Bank Group Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, spread on variable rate basis			1.00%
Wells Fargo Bank Group Credit Facility [Member] | Interest Rate Base Rate Option 1 [Member]
Line of Credit Facility [Line Items]
Debt Instrument Description Of Variable Rate Basis			an adjusted London Interbank Offered Rate ("LIBOR") plus a margin.
Wells Fargo Bank Group Credit Facility [Member] | Interest Rate Base Rate Option 1(a) [Member]
Line of Credit Facility [Line Items]
Debt Instrument Description Of Variable Rate Basis			prime commercial lending rate established by Wells Fargo Bank
Wells Fargo Bank Group Credit Facility [Member] | Interest Rate Base Rate Option 1(b) [Member]
Line of Credit Facility [Line Items]
Debt Instrument Description Of Variable Rate Basis			Federal Funds Rate plus 0.5% per annum
Wells Fargo Bank Group Credit Facility [Member] | Interest Rate Base Rate Option 1(c) [Member]
Line of Credit Facility [Line Items]
Debt Instrument Description Of Variable Rate Basis			one month LIBOR Rate plus 1.0% per annum
Wells Fargo Bank Group Credit Facility Tranche One [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment				200,000	200,000
Wells Fargo Bank Group Credit Facility Tranche Two [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment					600,000
Citibank Holdings Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, initiation date			Aug. 15, 2011
Line of credit facility, term of credit facility			3 years
Line of credit facility, commitment				150,000
Citibank Bilateral Agreement [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment				$ 300,000	$ 300,000
Per annum commitment fee on undrawn credit			0.15%
Citibank Bilateral Agreement [Member] | Interest Rate Base Rate Option 1(a) [Member]
Line of Credit Facility [Line Items]
Debt Instrument Description Of Variable Rate Basis			% per annum of the principal amount of issued standard letters of credit (expiry of 15 months or less)
Percentage of interest on the principal amount of letters of credit issued			0.35%
Maximum term allowed to execute issued letters of credit			15 months
Citibank Bilateral Agreement [Member] | Interest Rate Base Rate Option 1(b) [Member]
Line of Credit Facility [Line Items]
Debt Instrument Description Of Variable Rate Basis			% per annum of the principal amount of issued extended tenor letters of credit (expiry maximum of up to 60 months).
Percentage of interest on the principal amount of issued extended tenor letters of credit			0.45%
Maximum term allowed to execute issued letters of credit			60 months
Everest International Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, required minimum debt-to-capital ratio			0.35		0.35
Line of credit facility, required minimum net worth					$ 5,215,784
Consolidated net worth, percentage				70.00%
Percentage amount of consolidated net income required to be reserved to maintain compliance with facility covenants			25.00%
Percentage amount of increase in consolidated net worth from new issuance required to be reserved to maintain compliance with facility covenants			25.00%
Increase in reserves due to new issuance during the period required to maintain compliance with facility covenants					$ 5,468,960
Lloyd's of London Bank Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, term of credit facility			4 years
Line of credit facility, commitment | £			£ 175,000			£ 175,000
Percentage commitment fee per annum			0.10%
Per annum commitment fee on undrawn credit			0.35%