Investments - Narrative (Details) $ in Thousands		3 Months Ended			6 Months Ended
	Jan. 01, 2020 USD ($)	Jun. 30, 2020 USD ($) issuer	Jun. 30, 2019 USD ($) loan	Mar. 31, 2019 USD ($) security	Jun. 30, 2020 USD ($) issuer	Jun. 30, 2019 USD ($) loan	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) issuer loan	Dec. 31, 2018 USD ($)
Gain (Loss) on Securities [Line Items]
Number of issuers | issuer		56			56
Total fair value		$ 109,547			$ 109,547
Gross unrealized losses		3,135			3,135
Fixed maturity securities, available-for-sale		1,747,404			1,747,404			$ 1,433,626
Accounting Standards Update [Extensible List]						us-gaap:AccountingStandardsUpdate201602Member
Shareholders' equity		795,711	$ 791,050	$ 754,297	795,711	$ 791,050	$ 720,317	$ 778,581	$ 709,241
Bank loan participations		127,698			127,698
Unpaid principal balance		157,200			157,200
Net investment income		15,350	17,535		36,186	36,966
Net realized and unrealized losses on investments		(21,593)	$ (1,063)		36,814	$ (2,688)
Losses due to credit-related impairments		2,800			7,800
Number of impaired loans | loan			2			2		7
Carrying value of impaired loans			$ 3,400			$ 3,400		$ 6,900
Unpaid principal on impaired loans			3,600			3,600		14,300
Allowance for credit losses on impaired loans			231			231		7,200
Average recorded investment in impaired bank loans			1,700
Net realized gain (loss) on changes in fair value of impaired bank loans			(231)			(1,800)
Changes in fair values of bank loan participations		26,570	0		(17,377)	0
Bank Loan Participations
Gain (Loss) on Securities [Line Items]
Net investment income		3,100			7,300
Retained Earnings
Gain (Loss) on Securities [Line Items]
Shareholders' equity		61,770	$ 112,729	$ 101,617	61,770	$ 112,729	$ 35,412	89,586	79,753
Cumulative Effect, Period of Adoption, Adjustment
Gain (Loss) on Securities [Line Items]
Shareholders' equity									8,280
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
Gain (Loss) on Securities [Line Items]
Shareholders' equity									$ 8,280
Fixed maturity securities
Gain (Loss) on Securities [Line Items]
Total fair value								265,969
Gross unrealized losses								2,598
Fixed maturity securities, available-for-sale		1,747,404			1,747,404			1,433,626
Aggregate net unrealized loss		$ 3,135			$ 3,135			$ 2,598
Number of impaired fixed maturities | security				3
Impairment losses				$ 271
Bank Loan Portfolio
Gain (Loss) on Securities [Line Items]
Number of issuers with impaired loans | issuer								6
Loans from Issuer Experiencing Liquidity Concerns
Gain (Loss) on Securities [Line Items]
Number of impaired loans | loan								2
Number of issuers with impaired loans | issuer								1
Allowance for credit losses on impaired loans								$ 5,100
BBB- or better | Fixed maturity securities
Gain (Loss) on Securities [Line Items]
Percentage of available for sale securities		99.70%			99.70%
Below BBB-
Gain (Loss) on Securities [Line Items]
Fixed maturity securities, available-for-sale		$ 5,500			$ 5,500
Aggregate net unrealized loss		6			6
ASU 2019-05
Gain (Loss) on Securities [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201905Member
ASU 2019-05 | Cumulative Effect, Period of Adoption, Adjustment
Gain (Loss) on Securities [Line Items]
Bank loan participations	$ (8,400)							(8,400)
Shareholders' equity								(7,827)
ASU 2019-05 | Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
Gain (Loss) on Securities [Line Items]
Shareholders' equity	$ (7,800)							(7,827)
ASU 2016-13
Gain (Loss) on Securities [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
ASU 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
Gain (Loss) on Securities [Line Items]
Shareholders' equity								(265)
ASU 2016-13 | Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
Gain (Loss) on Securities [Line Items]
Shareholders' equity	$ (265)							$ (265)
Bank Holding Company
Gain (Loss) on Securities [Line Items]
Carrying value of collateralized loan obligation held		$ 1,500			$ 1,500