Employee Benefit Plans - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee benefit plans [Line Items]
Proceeds from option exercises	$ 6,000,000	$ 5,000,000	$ 12,000,000
Expected volatility historical share price period	3 years
Expected term	18 months
Recognized contribution expense	$ 12,000,000	$ 11,000,000	$ 11,000,000
Stock Options [Member]
Employee benefit plans [Line Items]
Options granted (in shares)	0	0	0
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options	$ 0
Aggregate intrinsic value of awards outstanding	$ 7,300,000
Weighted average remaining contractual term of awards outstanding (in years)	1 year 2 months 4 days
Aggregate intrinsic value of exercisable awards	$ 7,300,000
Weighted average remaining contractual term of exercisable awards (in years)	1 year 2 months 4 days
Total intrinsic value of awards exercised	$ 9,900,000	$ 6,600,000	$ 4,600,000
Proceeds from option exercises	$ 2,400,000	$ 4,700,000	$ 12,000,000
Stock Options [Member] | Employee [Member]
Employee benefit plans [Line Items]
Vesting period of awards (in years)	3 years
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
Vesting period of awards (in years)	3 years
Years from grant date until expiration	7 years
Aggregate intrinsic value of awards outstanding	$ 500,000
Weighted average remaining contractual term of awards outstanding (in years)	1 year 1 month 2 days
Aggregate intrinsic value of exercisable awards	$ 500,000
Weighted average remaining contractual term of exercisable awards (in years)	1 year 1 month 2 days
Total intrinsic value of awards exercised	$ 3,800,000	$ 700,000	$ 40,000
Proceeds from option exercises	$ 2,700,000	200,000	100,000
Period of highest average share price measurement on performance shares	40 days
Unrecognized compensation expense	$ 0
Time Vested and Performance Stock Options [Member]
Employee benefit plans [Line Items]
Tax benefit from stock options exercised	$ 1,500,000
Restricted Stock Awards [Member]
Employee benefit plans [Line Items]
Expected weighted-average remaining service period for recognition of unrecognized compensation cost (in years)	4 months 8 days
Unrecognized compensation expense	$ 700,000
Total fair value of awards vested	$ 1,900,000	1,500,000	1,600,000
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)	1 year 8 months 4 days
Unrecognized compensation expense	$ 15,200,000
Total fair value of awards delivered	$ 8,000,000	7,000,000	2,000,000
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 8 months 4 days
Unrecognized compensation expense	$ 9,900,000
Total fair value of awards delivered	$ 6,000,000	$ 8,000,000	$ 2,100,000
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)	600,000
Assured Guaranty Ltd. 2004 Long-Term Incentive Plan [Member]
Employee benefit plans [Line Items]
Maximum number of common shares that may be delivered	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	9,779,294
Performance Period Hurdle, One [Member]
Employee benefit plans [Line Items]
Vesting percentage	50.00%
Vesting from prior period, percentage	35.00%
Performance Period Hurdle, One [Member] | Performance Stock Options [Member]
Employee benefit plans [Line Items]
Vesting percentage	35.00%
Performance Period Hurdle, Two [Member] | Performance Stock Options [Member]
Employee benefit plans [Line Items]
Vesting percentage	50.00%
Performance Period Hurdle, Two [Member] | Performance Restricted Stock Units [Member]
Employee benefit plans [Line Items]
Vesting percentage	100.00%
Performance Period Hurdle, Three [Member] | Performance Stock Options [Member]
Employee benefit plans [Line Items]
Vesting percentage	100.00%
Performance Period Hurdle, Three [Member] | Performance Restricted Stock Units [Member]
Employee benefit plans [Line Items]
Vesting percentage	200.00%
United States
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
Employee benefit plans [Line Items]
Maximum amount that can be contributed by eligible participants	$ 18,500
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%