Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2019 USD ($) Holdings	Dec. 31, 2018 USD ($) Holdings
Investment grade
Fair value
Less than 12 months	$ 80,157	$ 242,821
12 months or longer	46,355	98,118
Total	126,512	340,939
Unrealized losses
Less than 12 months	176	1,295
12 months or longer	255	1,641
Total	$ 431	$ 2,936
No. of holdings | Holdings	85	147
Non-investment grade
Fair value
Less than 12 months	$ 28,262	$ 125,813
12 months or longer	9,985	19,987
Total	38,247	145,800
Unrealized losses
Less than 12 months	842	6,608
12 months or longer	1,131	1,686
Total	$ 1,973	$ 8,294
No. of holdings | Holdings	209	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	$ 39,871	$ 157,300
12 months or longer	30,518	86,586
Total	70,389	243,886
Unrealized losses
Less than 12 months	852	6,866
12 months or longer	1,179	2,688
Total	$ 2,031	$ 9,554
No. of holdings | Holdings	229	635
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 20,440	$ 777
12 months or longer	0	1,618
Total	20,440	2,395
Unrealized losses
Less than 12 months	55	6
12 months or longer	0	26
Total	$ 55	$ 32
No. of holdings | Holdings	7	3
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 7,325	$ 17,624
12 months or longer	538	16,997
Total	7,863	34,621
Unrealized losses
Less than 12 months	19	175
12 months or longer	2	412
Total	$ 21	$ 587
No. of holdings | Holdings	10	30
Collateralized debt obligations
Fair value
Less than 12 months	$ 39,202	$ 55,246
12 months or longer	25,284	1,248
Total	64,486	56,494
Unrealized losses
Less than 12 months	84	826
12 months or longer	205	4
Total	$ 289	$ 830
No. of holdings | Holdings	47	39
Other debt securities
Fair value
Less than 12 months	$ 1,581	$ 8,213
12 months or longer	0	0
Total	1,581	8,213
Unrealized losses
Less than 12 months	8	11
12 months or longer	0	0
Total	$ 8	$ 11
No. of holdings | Holdings	1	7
Available-for-sale securities
Fair value
Less than 12 months	$ 108,419	$ 368,634
12 months or longer	56,340	118,105
Total	164,759	486,739
Unrealized losses
Less than 12 months	1,018	7,903
12 months or longer	1,386	3,327
Total	$ 2,404	$ 11,230
No. of holdings | Holdings	294	721