Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2021 USD ($) holding	Dec. 31, 2020 USD ($) holding
Investment grade
Fair value
Less than 12 months	$ 196,591	$ 86,807
12 months or longer	16,711	10,899
Total	213,302	97,706
Unrealized losses
Less than 12 months	1,727	561
12 months or longer	257	99
Total	$ 1,984	$ 660
No. of holdings | holding	215	119
Non-investment grade
Fair value
Less than 12 months	$ 33,960	$ 28,686
12 months or longer	7,152	7,952
Total	41,112	36,638
Unrealized losses
Less than 12 months	281	593
12 months or longer	330	870
Total	$ 611	$ 1,463
No. of holdings | holding	308	251
Available-for-sale securities
Fair value
Less than 12 months	$ 230,551	$ 115,493
12 months or longer	23,863	18,851
Total	254,414	134,344
Unrealized losses
Less than 12 months	2,008	1,154
12 months or longer	587	969
Total	$ 2,595	$ 2,123
No. of holdings | holding	523	370
Corporate debt securities
Fair value
Less than 12 months	$ 96,735	$ 39,693
12 months or longer	10,396	7,952
Total	107,131	47,645
Unrealized losses
Less than 12 months	741	644
12 months or longer	402	870
Total	$ 1,143	$ 1,514
No. of holdings | holding	345	257
Collateralized debt obligations
Fair value
Less than 12 months	$ 52,441	$ 50,036
12 months or longer	12,194	10,899
Total	64,635	60,935
Unrealized losses
Less than 12 months	189	232
12 months or longer	46	99
Total	$ 235	$ 331
No. of holdings | holding	81	65
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 13,237	$ 16,582
12 months or longer	974	0
Total	14,211	16,582
Unrealized losses
Less than 12 months	326	235
12 months or longer	125	0
Total	$ 451	$ 235
No. of holdings | holding	31	31
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 63,636	$ 8,163
12 months or longer	299	0
Total	63,935	8,163
Unrealized losses
Less than 12 months	691	34
12 months or longer	14	0
Total	$ 705	$ 34
No. of holdings | holding	56	13
Other debt securities
Fair value
Less than 12 months	$ 3,163	$ 1,019
12 months or longer	0	0
Total	3,163	1,019
Unrealized losses
Less than 12 months	17	9
12 months or longer	0	0
Total	$ 17	$ 9
No. of holdings | holding	9	4
U.S. Treasury
Fair value
Less than 12 months	$ 1,339
12 months or longer	0
Total	1,339
Unrealized losses
Less than 12 months	44
12 months or longer	0
Total	$ 44
No. of holdings | holding	1