Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2020 USD ($) Holdings	Dec. 31, 2019 USD ($) Holdings
Investment grade
Fair value
Less than 12 months	$ 184,738	$ 76,315
12 months or longer	11,992	46,390
Total	196,730	122,705
Unrealized losses
Less than 12 months	1,916	287
12 months or longer	241	218
Total	$ 2,157	$ 505
No. of holdings | Holdings	178	100
Non-investment grade
Fair value
Less than 12 months	$ 47,484	$ 11,421
12 months or longer	8,236	10,691
Total	55,720	22,112
Unrealized losses
Less than 12 months	1,998	324
12 months or longer	1,436	1,359
Total	$ 3,434	$ 1,683
No. of holdings | Holdings	400	146
Available-for-sale securities
Fair value
Less than 12 months	$ 232,222	$ 87,736
12 months or longer	20,228	57,081
Total	252,450	144,817
Unrealized losses
Less than 12 months	3,914	611
12 months or longer	1,677	1,577
Total	$ 5,591	$ 2,188
No. of holdings | Holdings	578	246
Corporate debt securities
Fair value
Less than 12 months	$ 116,649	$ 25,804
12 months or longer	8,236	15,699
Total	124,885	41,503
Unrealized losses
Less than 12 months	2,493	342
12 months or longer	1,436	1,362
Total	$ 3,929	$ 1,704
No. of holdings | Holdings	437	158
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 17,274	$ 16,712
12 months or longer	0	0
Total	17,274	16,712
Unrealized losses
Less than 12 months	12	50
12 months or longer	0	0
Total	$ 12	$ 50
No. of holdings | Holdings	11	7
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 19,076	$ 21,981
12 months or longer	0	372
Total	19,076	22,353
Unrealized losses
Less than 12 months	393	147
12 months or longer	0	1
Total	$ 393	$ 148
No. of holdings | Holdings	29	30
Collateralized debt obligations
Fair value
Less than 12 months	$ 78,166	$ 20,889
12 months or longer	11,992	41,010
Total	90,158	61,899
Unrealized losses
Less than 12 months	998	33
12 months or longer	241	214
Total	$ 1,239	$ 247
No. of holdings | Holdings	97	49
Other debt securities
Fair value
Less than 12 months	$ 1,057	$ 2,350
12 months or longer	0	0
Total	1,057	2,350
Unrealized losses
Less than 12 months	18	39
12 months or longer	0	0
Total	$ 18	$ 39
No. of holdings | Holdings	4	2