Credit Facilities (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011
Line of Credit Facility [Line Items]
Line of credit facility, number of facilities	3
Line of credit facility, commitment	$ 1,250,000
Wells Fargo Bank Group Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, number of tranches	2
Line of credit facility, term of credit facility	4 years	5 years
Line of credit facility, commitment	800,000	850,000
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	4,470,565
Wells Fargo Bank Group Credit Facility [Member] | Interest Rate Base Rate Option 1(a) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, 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, description of variable rate basis	Federal Funds Rate plus 0.5% per annum
Line of credit facility, spread on variable rate basis	0.50%
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, description of variable rate basis	one month LIBOR Rate plus 1.0% per annum
Line of credit facility, spread on variable rate basis	1.00%
Wells Fargo Bank Group Credit Facility [Member] | Interest Rate Base Rate Option 2 [Member]
Line of Credit Facility [Line Items]
Line of credit facility, description of variable rate basis	adjusted London Interbank Offered Rate ("LIBOR") plus a margin
Wells Fargo Bank Group Credit Facility Tranche One [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment	200,000	350,000
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, term of credit facility	3 years	5 years
Line of credit facility, commitment	150,000	150,000
Line of credit facility, required minimum debt-to-capital ratio	0.35
Line of credit facility, required minimum net worth	1,875,000
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	1,990,497
Citibank Holdings Credit Facility [Member] | Interest Rate Base Rate Option 1(a) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, description of variable rate basis	rate of interest publicly announced by Citibank as its base rate
Citibank Holdings Credit Facility [Member] | Interest Rate Base Rate Option 1(b) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, description of variable rate basis	% per annum above the Federal Funds Rate
Line of credit facility, spread on variable rate basis	0.50%
Citibank Holdings Credit Facility [Member] | Interest Rate Base Rate Option 1(c) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, description of variable rate basis	% above the one month LIBOR, in each case plus the applicable margin
Line of credit facility, spread on variable rate basis	1.00%
Citibank Holdings Credit Facility [Member] | Interest Rate Base Rate Option 2 [Member]
Line of Credit Facility [Line Items]
Line of credit facility, description of variable rate basis	periodic fixed rate equal to the Eurodollar Rate plus an applicable margin
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]
Line of credit facility, 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]
Line of credit facility, 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