Employee Benefit Plans - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee benefit plans [Line Items]
Options, excluding performance stock options, vesting period (in years)	3 years
Recognized contribution expense	$ 12,000,000	$ 12,000,000	$ 11,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	18 months
Stock Options [Member]
Employee benefit plans [Line Items]
Options granted (in shares)	0	0	0
Share-based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount	$ 0
Aggregate intrinsic value of awards outstanding	$ 2,600,000
Weighted average remaining contractual term of awards outstanding (in years)	8 months 12 days
Aggregate intrinsic value of exercisable awards	$ 2,600,000
Weighted average remaining contractual term of exercisable awards (in years)	8 months 12 days
Total intrinsic value of awards exercised	$ 8,200,000	$ 9,900,000	$ 6,600,000
Proceeds from option exercises	$ 2,300,000	$ 2,400,000	$ 4,700,000
Stock Options [Member] | Employee [Member] | Minimum [Member]
Employee benefit plans [Line Items]
Years from grant date until expiration	7 years
Stock Options [Member] | Employee [Member] | Maximum [Member]
Employee benefit plans [Line Items]
Years from grant date until expiration	10 years
Stock Options [Member] | Directors [Member}
Employee benefit plans [Line Items]
Vesting period of awards (in years)	1 year
Stock Options [Member] | Directors [Member} | Minimum [Member]
Employee benefit plans [Line Items]
Years from grant date until expiration	7 years
Stock Options [Member] | Directors [Member} | Maximum [Member]
Employee benefit plans [Line Items]
Years from grant date until expiration	10 years
Performance Stock Options [Member]
Employee benefit plans [Line Items]
Options granted (in shares)	0	0	0
Years from grant date until expiration	7 years
Total intrinsic value of awards exercised	$ 700,000	$ 3,800,000	$ 700,000
Proceeds from option exercises	$ 500,000	2,700,000	200,000
Period of highest average share price measurement on performance shares	40 days
Time Vested and Performance Stock Options [Member]
Employee benefit plans [Line Items]
Tax benefit from stock options exercised	$ 900,000
Restricted Stock Awards [Member]
Employee benefit plans [Line Items]
Expected weighted-average remaining service period for recognition of unrecognized compensation cost (in years)	3 months 18 days
Unrecognized compensation expense	$ 700,000
Total fair value of awards vested	$ 1,800,000	1,900,000	1,500,000
Granted (in dollars per share)	$ 45.98
Restricted Stock Awards [Member] | Employee [Member]
Employee benefit plans [Line Items]
Vesting period of awards (in years)	4 years
Restricted Stock Awards [Member] | Directors [Member}
Employee benefit plans [Line Items]
Vesting period of awards (in years)	1 year
Restricted Stock Units (RSUs) [Member]
Employee benefit plans [Line Items]
Expected weighted-average remaining service period for recognition of unrecognized compensation cost (in years)	2 years 6 months
Unrecognized compensation expense	$ 25,000,000
Total fair value of awards delivered	$ 11,000,000	8,000,000	7,000,000
Granted (in dollars per share)	$ 44.40
Performance Restricted Stock Units [Member]
Employee benefit plans [Line Items]
Vesting period of awards (in years)	3 years
Expected weighted-average remaining service period for recognition of unrecognized compensation cost (in years)	1 year 9 months 18 days
Vesting percentage	100.00%
Unrecognized compensation expense	$ 10,000,000
Total fair value of awards delivered	$ 6,000,000	$ 6,000,000	$ 8,000,000
Granted (in dollars per share)	$ 44.00	$ 12.66
Performance Restricted Stock Units [Member] | Minimum [Member]
Employee benefit plans [Line Items]
Vesting from prior period, percentage	0.00%
Performance Restricted Stock Units, Tied to Relative TSR [Member]
Employee benefit plans [Line Items]
Granted (in dollars per share)	$ 46.66
Employee Stock Purchase Plan [Member] | Employee Stock [Member]
Employee benefit plans [Line Items]
Value of shares that can be purchased as a percentage of participant's compensation	10.00%
Value of shares that can be purchased	$ 25,000
Purchase price of shares as a percentage of the fair market value of the stock	85.00%
Capital shares reserved for future issuance (in shares)	850,000
Assured Guaranty Ltd. 2004 Long-Term Incentive Plan [Member]
Employee benefit plans [Line Items]
Maximum number of common shares that may be delivered (in shares)	18,670,000
Assured Guaranty Ltd. 2004 Long-Term Incentive Plan [Member] | Stock Options [Member]
Employee benefit plans [Line Items]
Number of common shares available for grant (in shares)	9,311,090
AGL common stock performance based [Member] | Performance Stock Options [Member]
Employee benefit plans [Line Items]
Vesting percentage	35.00%
AGL common stock performance based [Member] | Performance Restricted Stock Units [Member] | Maximum [Member]
Employee benefit plans [Line Items]
Vesting from prior period, percentage	200.00%
Total shareholder return based [Member] | Performance Stock Options [Member]
Employee benefit plans [Line Items]
Vesting percentage	50.00%
Total shareholder return based [Member] | Performance Restricted Stock Units [Member] | Maximum [Member]
Employee benefit plans [Line Items]
Vesting percentage	250.00%
Adjusted book value growth based [Member] | Performance Stock Options [Member]
Employee benefit plans [Line Items]
Vesting percentage	100.00%
Adjusted book value growth based [Member] | Performance Restricted Stock Units [Member] | Maximum [Member]
Employee benefit plans [Line Items]
Vesting percentage	200.00%
United States [Member]
Employee benefit plans [Line Items]
Core contribution made by the company as a percentage of participant's compensation	6.00%
Number of years of service to become fully vested	1 year
401 (k) Plan [Member] | United States [Member]
Employee benefit plans [Line Items]
Percentage by which employer contribution matches up to 6% of participant's compensation	100.00%
Maximum percentage of participant's compensation eligible for employer contribution match	6.00%