Related Party Transactions	12 Months Ended
Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions Two of the Company's investment portfolio managers, Wellington Management Company, LLP (Wellington) and BlackRock Financial Management, Inc. (BlackRock), each own more than 5% of the Company's common shares. In addition, the Company has a minority interest in Wasmer, Schroder & Company LLC, which is also one of the Company's investment portfolio managers. The investment management expense from transactions with these related parties was approximately $4.0 million in 2018, $4.1 million in 2017 and $4.2 million in 2016. In addition, the Company recognized $1.2 million in 2018 in income from its investment in Wasmer, Schroder & Company LLC. As of December 31, 2018 and 2017 there were no other significant amounts payable to or amounts receivable from related parties, other than compensation in the ordinary course of business. The Company used a portion of its share repurchase program to repurchase 297,131 common shares from its Chief Executive Officer and 23,062 common shares from its then General Counsel on January 6, 2017. The shares were purchased at the closing price of a common share of the Company on the New York Stock Exchange on January 6, 2017. Separately, these officers also received 297,131 and 23,062 common shares, respectively, on January 6, 2017 in settlement of 297,131 share units and 23,062 share units held by them in the employer stock fund of the Assured Guaranty Ltd. Supplemental Employee Retirement Plan (the AGL SERP). The distribution of shares occurred in January 2017 pursuant to the terms of an amendment adopted in 2011 to the AGL SERP. Such amendment was adopted to comply with requirements of Section 409A of the Code and Section 457A of the Code, which required all grandfathered amounts (within the meaning of Section 457A of the Code), including the units in the employer stock fund in the AGL SERP, to be included in the income of the applicable participant no later than 2017.

Related Party Transactions

Two of the Company's investment portfolio managers, Wellington Management Company, LLP (Wellington) and BlackRock Financial Management, Inc. (BlackRock), each own more than 5% of the Company's common shares. In addition, the Company has a minority interest in Wasmer, Schroder & Company LLC, which is also one of the Company's investment portfolio managers. The investment management expense from transactions with these related parties was approximately $4.0 million in 2018, $4.1 million in 2017 and $4.2 million in 2016. In addition, the Company recognized $1.2 million in 2018 in income from its investment in Wasmer, Schroder & Company LLC. As of December 31, 2018 and 2017 there were no other significant amounts payable to or amounts receivable from related parties, other than compensation in the ordinary course of business.

The Company used a portion of its share repurchase program to repurchase 297,131 common shares from its Chief Executive Officer and 23,062 common shares from its then General Counsel on January 6, 2017. The shares were purchased at the closing price of a common share of the Company on the New York Stock Exchange on January 6, 2017. Separately, these officers also received 297,131 and 23,062 common shares, respectively, on January 6, 2017 in settlement of 297,131 share units and 23,062 share units held by them in the employer stock fund of the Assured Guaranty Ltd. Supplemental Employee Retirement Plan (the AGL SERP). The distribution of shares occurred in January 2017 pursuant to the terms of an amendment adopted in 2011 to the AGL SERP. Such amendment was adopted to comply with requirements of Section 409A of the Code and Section 457A of the Code, which required all grandfathered amounts (within the meaning of Section 457A of the Code), including the units in the employer stock fund in the AGL SERP, to be included in the income of the applicable participant no later than 2017.