Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2019 USD ($) Holdings	Dec. 31, 2018 USD ($) Holdings
Investment grade
Fair value
Less than 12 months	$ 136,432	$ 242,821
12 months or longer	59,023	98,118
Total	195,455	340,939
Unrealized losses
Less than 12 months	296	1,295
12 months or longer	354	1,641
Total	$ 650	$ 2,936
No. of holdings | Holdings	111	147
Non-investment grade
Fair value
Less than 12 months	$ 31,454	$ 125,813
12 months or longer	15,032	19,987
Total	46,486	145,800
Unrealized losses
Less than 12 months	740	6,608
12 months or longer	971	1,686
Total	$ 1,711	$ 8,294
No. of holdings | Holdings	273	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	$ 104,809	$ 157,300
12 months or longer	58,666	86,586
Total	163,475	243,886
Unrealized losses
Less than 12 months	844	6,866
12 months or longer	1,105	2,688
Total	$ 1,949	$ 9,554
No. of holdings | Holdings	334	635
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 15,710	$ 777
12 months or longer	0	1,618
Total	15,710	2,395
Unrealized losses
Less than 12 months	21	6
12 months or longer	0	26
Total	$ 21	$ 32
No. of holdings | Holdings	2	3
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 6,965	$ 17,624
12 months or longer	547	16,997
Total	7,512	34,621
Unrealized losses
Less than 12 months	6	175
12 months or longer	4	412
Total	$ 10	$ 587
No. of holdings | Holdings	7	30
Collateralized debt obligations
Fair value
Less than 12 months	$ 40,402	$ 55,246
12 months or longer	14,842	1,248
Total	55,244	56,494
Unrealized losses
Less than 12 months	165	826
12 months or longer	216	4
Total	$ 381	$ 830
No. of holdings | Holdings	41	39
Other debt securities
Fair value
Less than 12 months		$ 8,213
12 months or longer		0
Total		8,213
Unrealized losses
Less than 12 months		11
12 months or longer		0
Total		$ 11
No. of holdings | Holdings		7
Available-for-sale securities
Fair value
Less than 12 months	$ 167,886	$ 368,634
12 months or longer	74,055	118,105
Total	241,941	486,739
Unrealized losses
Less than 12 months	1,036	7,903
12 months or longer	1,325	3,327
Total	$ 2,361	$ 11,230
No. of holdings | Holdings	384	721