Credit Facilities (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	0 Months Ended	12 Months Ended
	Jul. 27, 2007	Dec. 31, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Line of credit facility, number of facilities		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	5 years	4 years
Line of credit facility, commitment	850,000	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		5,115,795
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]
Line of credit facility, spread on variable rate basis		0.50%
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]
Line of credit facility, spread on variable rate basis		1.00%
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, Expiration Date		Aug. 15, 2014
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, initiation date		Dec. 31, 2014
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