Credit Facilities (Narrative) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Aug. 15, 2014 USD ($)
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, number of tranches | item	2
Line of credit facility, term of credit facility	4 years
Line of credit facility, commitment	$ 800,000	$ 800,000
Line of credit facility, required minimum debt-to-capital ratio	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	$ 5,252,761
Wells Fargo Bank Group Credit Facility [Member] | Before Amended And Restated Agreement [Member]
Line of Credit Facility [Line Items]
Line of credit facility, term of credit facility	5 years
Line of credit facility, commitment	$ 850,000
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 One [Member] | Interest Rate Base Rate Option 1(b) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, spread on variable rate basis	0.50%
Wells Fargo Bank Group Credit Facility Tranche One [Member] | Interest Rate Base Rate Option 1(c) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, spread on variable rate basis	1.00%
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, 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]
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]
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