Accounting Policies - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) company	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) company	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Pre-tax (income) loss	$ (39,760)	$ (24,962)	$ 1,458	$ (50,453)
Income tax expense (benefit)	4,146	4,655	(257)	$ 7,418
Effective U.S. federal income tax expense rate				14.70%
Shareholders' equity	795,711	791,050	795,711	$ 791,050	$ 720,317		$ 778,581	$ 754,297	$ 709,241
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity									8,280
VIE, not primary beneficiary
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Investment in variable interest entities	$ 31,300		$ 31,300				31,200
UNITED STATES
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of insurance companies | company	5		5
BERMUDA
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of insurance companies | company	2		2
ASU 2019-05 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Bank loan participations						$ (8,400)	(8,400)
Shareholders' equity							(7,827)
ASU 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity							(265)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity	$ 61,770	$ 112,729	$ 61,770	$ 112,729	$ 35,412		89,586	$ 101,617	79,753
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity									$ 8,280
Retained Earnings | ASU 2019-05 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						(7,800)	(7,827)
Retained Earnings | ASU 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						$ (265)	$ (265)