Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2021 USD ($) holding	Dec. 31, 2020 USD ($) holding
Investment grade
Fair value
Less than 12 months	$ 194,135	$ 86,807
12 months or longer	26,334	10,899
Total	220,469	97,706
Unrealized losses
Less than 12 months	2,165	561
12 months or longer	135	99
Total	$ 2,300	$ 660
No. of holdings | holding	220	119
Non-investment grade
Fair value
Less than 12 months	$ 32,081	$ 28,686
12 months or longer	11,513	7,952
Total	43,594	36,638
Unrealized losses
Less than 12 months	363	593
12 months or longer	724	870
Total	$ 1,087	$ 1,463
No. of holdings | holding	354	251
Available-for-sale securities
Fair value
Less than 12 months	$ 226,216	$ 115,493
12 months or longer	37,847	18,851
Total	264,063	134,344
Unrealized losses
Less than 12 months	2,528	1,154
12 months or longer	859	969
Total	$ 3,387	$ 2,123
No. of holdings | holding	574	370
Corporate debt securities
Fair value
Less than 12 months	$ 123,931	$ 39,693
12 months or longer	11,703	7,952
Total	135,634	47,645
Unrealized losses
Less than 12 months	1,345	644
12 months or longer	726	870
Total	$ 2,071	$ 1,514
No. of holdings | holding	399	257
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 31,246	$ 8,163
12 months or longer	0	0
Total	31,246	8,163
Unrealized losses
Less than 12 months	411	34
12 months or longer	0	0
Total	$ 411	$ 34
No. of holdings | holding	41	13
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 23,116	$ 16,582
12 months or longer	1,186	0
Total	24,302	16,582
Unrealized losses
Less than 12 months	463	235
12 months or longer	13	0
Total	$ 476	$ 235
No. of holdings | holding	60	31
Collateralized debt obligations
Fair value
Less than 12 months	$ 34,317	$ 50,036
12 months or longer	24,958	10,899
Total	59,275	60,935
Unrealized losses
Less than 12 months	126	232
12 months or longer	120	99
Total	$ 246	$ 331
No. of holdings | holding	62	65
Other debt securities
Fair value
Less than 12 months	$ 7,330	$ 1,019
12 months or longer	0	0
Total	7,330	1,019
Unrealized losses
Less than 12 months	56	9
12 months or longer	0	0
Total	$ 56	$ 9
No. of holdings | holding	10	4
U.S. Treasury
Fair value
Less than 12 months	$ 6,276
12 months or longer	0
Total	6,276
Unrealized losses
Less than 12 months	127
12 months or longer	0
Total	$ 127
No. of holdings | holding	2