Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2020 USD ($) Holdings	Dec. 31, 2019 USD ($) Holdings
Investment grade
Fair value
Less than 12 months	$ 362,145	$ 76,315
12 months or longer	13,122	46,390
Total	375,267	122,705
Unrealized losses
Less than 12 months	19,158	287
12 months or longer	1,146	218
Total	$ 20,304	$ 505
No. of holdings | Holdings	298	100
Non-investment grade
Fair value
Less than 12 months	$ 78,906	$ 11,421
12 months or longer	6,822	10,691
Total	85,728	22,112
Unrealized losses
Less than 12 months	11,246	324
12 months or longer	2,917	1,359
Total	$ 14,163	$ 1,683
No. of holdings | Holdings	543	146
Available-for-sale securities
Fair value
Less than 12 months	$ 441,051	$ 87,736
12 months or longer	19,944	57,081
Total	460,995	144,817
Unrealized losses
Less than 12 months	30,404	611
12 months or longer	4,063	1,577
Total	$ 34,467	$ 2,188
No. of holdings | Holdings	841	246
Corporate debt securities
Fair value
Less than 12 months	$ 299,653	$ 25,804
12 months or longer	6,822	15,699
Total	306,475	41,503
Unrealized losses
Less than 12 months	20,247	342
12 months or longer	2,918	1,362
Total	$ 23,165	$ 1,704
No. of holdings | Holdings	662	158
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 2,798	$ 16,712
12 months or longer	0	0
Total	2,798	16,712
Unrealized losses
Less than 12 months	413	50
12 months or longer	0	0
Total	$ 413	$ 50
No. of holdings | Holdings	4	7
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 59,786	$ 21,981
12 months or longer	0	372
Total	59,786	22,353
Unrealized losses
Less than 12 months	3,729	147
12 months or longer	0	1
Total	$ 3,729	$ 148
No. of holdings | Holdings	73	30
Collateralized debt obligations
Fair value
Less than 12 months	$ 72,456	$ 20,889
12 months or longer	13,122	41,010
Total	85,578	61,899
Unrealized losses
Less than 12 months	5,490	33
12 months or longer	1,145	214
Total	$ 6,635	$ 247
No. of holdings | Holdings	87	49
Other debt securities
Fair value
Less than 12 months	$ 6,358	$ 2,350
12 months or longer	0	0
Total	6,358	2,350
Unrealized losses
Less than 12 months	525	39
12 months or longer	0	0
Total	$ 525	$ 39
No. of holdings | Holdings	15	2