Financial Guaranty Contracts Accounted for as Credit Derivatives - Credit Derivatives Subordination and Ratings (Details) - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	[1]	$ 19,959.0	$ 25,594.0
Pooled corporate obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 12,274.0	$ 17,523.0
Original Subordination (as a percent)	[2]	26.60%	29.20%
Current Subordination (as a percent)	[2]	28.40%	32.30%
Pooled corporate obligations [Member] | Collateralized loan obligations and collateral bond obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 2,583.0	$ 5,873.0
Original Subordination (as a percent)	[2]	29.20%	30.90%
Current Subordination (as a percent)	[2]	45.30%	42.30%
Pooled corporate obligations [Member] | Synthetic investment grade pooled corporate [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 7,920.0	$ 7,108.0
Original Subordination (as a percent)	[2]	22.20%	21.70%
Current Subordination (as a percent)	[2]	19.90%	19.40%
Pooled corporate obligations [Member] | TruPS CDOs [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 1,771.0	$ 3,429.0
Original Subordination (as a percent)	[2]	42.20%	45.80%
Current Subordination (as a percent)	[2]	41.60%	42.60%
Pooled corporate obligations [Member] | Market Value of CDOs of corporate obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 0.0	$ 1,113.0
Original Subordination (as a percent)	[2]	0.00%	17.00%
Current Subordination (as a percent)	[2]	0.00%	30.10%
RMBS [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 1,182.0	$ 1,526.0
Original Subordination (as a percent)	[2]	25.10%	24.10%
Current Subordination (as a percent)	[2]	40.30%	37.40%
RMBS [Member] | Option Adjustable Rate Mortgage and Alt-A Mortgage [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 277.0	$ 351.0
Original Subordination (as a percent)	[2]	9.40%	10.50%
Current Subordination (as a percent)	[2]	12.70%	12.70%
RMBS [Member] | Subprime [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 891.0	$ 981.0
Original Subordination (as a percent)	[2]	27.80%	27.70%
Current Subordination (as a percent)	[2]	45.00%	45.20%
RMBS [Member] | Prime [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 0.0	$ 177.0
Original Subordination (as a percent)	[2]	0.00%	10.90%
Current Subordination (as a percent)	[2]	0.00%	0.00%
RMBS [Member] | Closed-end [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 14.0	$ 17.0
Original Subordination (as a percent)	[2]	0.00%	0.00%
Current Subordination (as a percent)	[2]	0.00%	0.00%
CMBS [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 159.0	$ 530.0
Original Subordination (as a percent)	[2]	56.90%	44.80%
Current Subordination (as a percent)	[2]	78.90%	52.60%
Other [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 6,344.0	$ 6,015.0
Original Subordination (as a percent)	[2]	0.00%	0.00%
Current Subordination (as a percent)	[2]	0.00%	0.00%
CIFG Holding Inc. [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 2.3

[1] The September 30, 2016 total amount includes $2.3 million net par outstanding of credit derivatives acquired from CIFG.

[2] Represents the sum of subordinate tranches and over-collateralization and does not include any benefit from excess interest collections that may be used to absorb losses.