Investments - Narrative (Details) $ in Thousands		3 Months Ended			9 Months Ended
	Jan. 01, 2020 USD ($)	Sep. 30, 2020 USD ($) issuer	Sep. 30, 2019 USD ($) issuer loan	Mar. 31, 2019 USD ($) security	Sep. 30, 2020 USD ($) issuer	Sep. 30, 2019 USD ($) issuer loan	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) issuer loan	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Gain (Loss) on Securities [Line Items]
Number of issuers | issuer		27			27
Total fair value		$ 138,100			$ 138,100
Gross unrealized losses		2,600			2,600
Fixed maturity securities, available-for-sale		1,813,471			1,813,471			$ 1,433,626
Accounting Standards Update [Extensible List]						us-gaap:AccountingStandardsUpdate201602Member
Shareholders' equity		821,406	$ 768,969		821,406	$ 768,969	$ 795,711	$ 778,581	$ 791,050	$ 709,241
Bank loan participations		131,198			131,198
Unpaid principal balance		151,800			151,800
Net investment income		14,959	17,878		51,145	54,844
Changes in fair values of bank loan participations		9,741	$ 0		(7,636)	$ 0
Losses due to credit-related impairments		437			8,200
Number of impaired loans | loan			5			5		7
Carrying value of impaired loans			$ 6,400			$ 6,400		$ 6,900
Unpaid principal on impaired loans			12,300			12,300		14,300
Allowance for credit losses on impaired loans			5,900			5,900		7,200
Average recorded investment in impaired bank loans						3,200
Investment income during period loans were impaired						(23)
Net realized gain (loss) on changes in fair value of impaired bank loans			(5,900)			(7,700)
Bank Loan Participations
Gain (Loss) on Securities [Line Items]
Net investment income		2,400			9,700
Retained Earnings
Gain (Loss) on Securities [Line Items]
Shareholders' equity		78,761	$ 78,344		78,761	$ 78,344	$ 61,770	89,586	$ 112,729	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		138,081			138,081			265,969
Gross unrealized losses		2,619			2,619			2,598
Fixed maturity securities, available-for-sale		1,813,471			1,813,471			1,433,626
Aggregate net unrealized gain		$ 91,529			$ 91,529			$ 37,691
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			2		2
Number of issuers with impaired loans | issuer			1			1		1
Allowance for credit losses on impaired loans			$ 4,200			$ 4,200		$ 5,100
BBB- or better | Fixed maturity securities
Gain (Loss) on Securities [Line Items]
Percentage of available for sale securities		99.50%			99.50%
Below BBB-
Gain (Loss) on Securities [Line Items]
Fixed maturity securities, available-for-sale		$ 8,800			$ 8,800
Aggregate net unrealized gain		56			56
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		$ 677			$ 677