Employee Benefit Plans - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) installments shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares
Employee benefit plans [Line Items]
Proceeds from option exercises	$ 5,000,000	$ 12,000,000	$ 5,000,000
Expected volatility historical share price period	3 years
Expected term	18 months
Recognized contribution expense	$ 11,000,000	$ 11,000,000	$ 10,000,000
Stock Options [Member]
Employee benefit plans [Line Items]
Options granted (in shares) | shares	0	0	0
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options	$ 0
Aggregate intrinsic value of awards outstanding	$ 14,000,000
Weighted average remaining contractual term of awards outstanding (in years)	1 year 4 months 8 days
Aggregate intrinsic value of exercisable awards	$ 14,000,000
Weighted average remaining contractual term of exercisable awards (in years)	1 year 4 months 8 days
Total intrinsic value of awards exercised	$ 6,600,000	$ 4,600,000	$ 2,800,000
Proceeds from option exercises	4,700,000	$ 12,000,000	$ 4,900,000
Tax benefit from stock options exercised	$ 1,800,000
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) | 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	$ 3,000,000
Weighted average remaining contractual term of awards outstanding (in years)	1 year 3 months 6 days
Aggregate intrinsic value of exercisable awards	$ 3,000,000
Weighted average remaining contractual term of exercisable awards (in years)	1 year 3 months 6 days
Total intrinsic value of awards exercised	$ 699,000	$ 41,000	$ 75,000
Proceeds from option exercises	$ 206,000	106,000	98,000
Period of highest average share price measurement on performance shares	40 days
Unrecognized compensation expense	$ 0
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	$ 1,500,000	1,600,000	1,000,000
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 9 months 6 days
Unrecognized compensation expense	$ 12,700,000
Total fair value of awards delivered	$ 7,000,000	2,000,000	6,000,000
Restricted Stock Units (RSUs) [Member] | Directors [Member}
Employee benefit plans [Line Items]
Vesting period of awards (in years)	1 year
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 6 days
Period of highest average share price measurement on performance shares	40 days
Unrecognized compensation expense	$ 8,800,000
Total fair value of awards delivered	$ 8,000,000	2,000,000	6,000,000
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 | 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 | 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 | shares	10,034,895
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 [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
United States [Member] | 401 (k) Plan [Member]
Employee benefit plans [Line Items]
Maximum amount that can be contributed by eligible participants	$ 18,000
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%
Performance Retention Plan [Member]
Employee benefit plans [Line Items]
Number of installments for award vesting | installments	3
Vesting period	4 years
Defined contribution expenses	$ 12,000,000	$ 12,000,000	$ 11,000,000