Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2019 USD ($) Holdings	Dec. 31, 2018 USD ($) Holdings
Investment grade
Fair value
Less than 12 months	$ 77,871	$ 242,821
12 months or longer	94,217	98,118
Total	172,088	340,939
Unrealized losses
Less than 12 months	534	1,295
12 months or longer	807	1,641
Total	$ 1,341	$ 2,936
No. of holdings | Holdings	124	147
Non-investment grade
Fair value
Less than 12 months	$ 75,201	$ 125,813
12 months or longer	34,566	19,987
Total	109,767	145,800
Unrealized losses
Less than 12 months	1,602	6,608
12 months or longer	1,446	1,686
Total	$ 3,048	$ 8,294
No. of holdings | Holdings	426	574
U.S. Treasury
Fair value
Less than 12 months	$ 0	$ 129,474
12 months or longer	11,748	11,656
Total	11,748	141,130
Unrealized losses
Less than 12 months	0	19
12 months or longer	100	197
Total	$ 100	$ 216
No. of holdings | Holdings	4	7
Corporate debt securities
Fair value
Less than 12 months	$ 82,879	$ 157,300
12 months or longer	94,393	86,586
Total	177,272	243,886
Unrealized losses
Less than 12 months	1,647	6,866
12 months or longer	1,867	2,688
Total	$ 3,514	$ 9,554
No. of holdings | Holdings	471	635
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 1,208	$ 777
12 months or longer	388	1,618
Total	1,596	2,395
Unrealized losses
Less than 12 months	4	6
12 months or longer	5	26
Total	$ 9	$ 32
No. of holdings | Holdings	2	3
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 9,491	$ 17,624
12 months or longer	19,023	16,997
Total	28,514	34,621
Unrealized losses
Less than 12 months	17	175
12 months or longer	232	412
Total	$ 249	$ 587
No. of holdings | Holdings	27	30
Collateralized debt obligations
Fair value
Less than 12 months	$ 59,394	$ 55,246
12 months or longer	3,231	1,248
Total	62,625	56,494
Unrealized losses
Less than 12 months	468	826
12 months or longer	49	4
Total	$ 517	$ 830
No. of holdings | Holdings	45	39
Other debt securities
Fair value
Less than 12 months	$ 100	$ 8,213
12 months or longer	0	0
Total	100	8,213
Unrealized losses
Less than 12 months	0	11
12 months or longer	0	0
Total	$ 0	$ 11
No. of holdings | Holdings	1	7
Available-for-sale securities
Fair value
Less than 12 months	$ 153,072	$ 368,634
12 months or longer	128,783	118,105
Total	281,855	486,739
Unrealized losses
Less than 12 months	2,136	7,903
12 months or longer	2,253	3,327
Total	$ 4,389	$ 11,230
No. of holdings | Holdings	550	721