Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2019 USD ($) Holdings	Dec. 31, 2018 USD ($) Holdings
Investment grade
Fair value
Less than 12 months	$ 76,315	$ 242,821
12 months or longer	46,390	98,118
Total	122,705	340,939
Unrealized losses
Less than 12 months	287	1,295
12 months or longer	218	1,641
Total	$ 505	$ 2,936
No. of holdings | Holdings	100	147
Non-investment grade
Fair value
Less than 12 months	$ 11,421	$ 125,813
12 months or longer	10,691	19,987
Total	22,112	145,800
Unrealized losses
Less than 12 months	324	6,608
12 months or longer	1,359	1,686
Total	$ 1,683	$ 8,294
No. of holdings | Holdings	146	574
U.S. Treasury
Fair value
Less than 12 months		$ 129,474
12 months or longer		11,656
Total		141,130
Unrealized losses
Less than 12 months		19
12 months or longer		197
Total		$ 216
No. of holdings | Holdings		7
Corporate debt securities
Fair value
Less than 12 months	$ 25,804	$ 157,300
12 months or longer	15,699	86,586
Total	41,503	243,886
Unrealized losses
Less than 12 months	342	6,866
12 months or longer	1,362	2,688
Total	$ 1,704	$ 9,554
No. of holdings | Holdings	158	635
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 16,712	$ 777
12 months or longer	0	1,618
Total	16,712	2,395
Unrealized losses
Less than 12 months	50	6
12 months or longer	0	26
Total	$ 50	$ 32
No. of holdings | Holdings	7	3
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 21,981	$ 17,624
12 months or longer	372	16,997
Total	22,353	34,621
Unrealized losses
Less than 12 months	147	175
12 months or longer	1	412
Total	$ 148	$ 587
No. of holdings | Holdings	30	30
Collateralized debt obligations
Fair value
Less than 12 months	$ 20,889	$ 55,246
12 months or longer	41,010	1,248
Total	61,899	56,494
Unrealized losses
Less than 12 months	33	826
12 months or longer	214	4
Total	$ 247	$ 830
No. of holdings | Holdings	49	39
Other debt securities
Fair value
Less than 12 months	$ 2,350	$ 8,213
12 months or longer	0	0
Total	2,350	8,213
Unrealized losses
Less than 12 months	39	11
12 months or longer	0	0
Total	$ 39	$ 11
No. of holdings | Holdings	2	7
Available-for-sale securities
Fair value
Less than 12 months	$ 87,736	$ 368,634
12 months or longer	57,081	118,105
Total	144,817	486,739
Unrealized losses
Less than 12 months	611	7,903
12 months or longer	1,577	3,327
Total	$ 2,188	$ 11,230
No. of holdings | Holdings	246	721