Investments - Gross Unrealized Loss Positions (Details) $ in Thousands	Dec. 31, 2018 USD ($) Holdings	Dec. 31, 2017 USD ($) Holdings
Fair value
Less than 12 months	$ 368,634	$ 272,033
Less than 12 months		282,770
12 months or longer	118,105	66,019
12 months or longer		66,019
Total	486,739	338,052
Total		348,789
Unrealized losses
Less than 12 months	7,903	2,138
Less than 12 months		2,238
12 months or longer	3,327	1,407
12 months or longer		1,407
Total	$ 11,230	3,545
Total		$ 3,645
No. of holdings | Holdings	721	411
No. of holdings | Holdings		417
Investment grade
Fair value
Less than 12 months	$ 242,821	$ 214,586
12 months or longer	98,118	62,193
Total	340,939	276,779
Unrealized losses
Less than 12 months	1,295	1,064
12 months or longer	1,641	985
Total	$ 2,936	$ 2,049
No. of holdings | Holdings	147	158
Non-investment grade
Fair value
Less than 12 months	$ 125,813	$ 57,447
12 months or longer	19,987	3,826
Total	145,800	61,273
Unrealized losses
Less than 12 months	6,608	1,074
12 months or longer	1,686	422
Total	$ 8,294	$ 1,496
No. of holdings | Holdings	574	253
U.S. Treasury
Fair value
Less than 12 months	$ 129,474	$ 10,237
12 months or longer	11,656	1,497
Total	141,130	11,734
Unrealized losses
Less than 12 months	19	110
12 months or longer	197	29
Total	$ 216	$ 139
No. of holdings | Holdings	7	4
States and political subdivisions
Fair value
Less than 12 months		$ 52,553
12 months or longer		14,361
Total		66,914
Unrealized losses
Less than 12 months		288
12 months or longer		332
Total		$ 620
No. of holdings | Holdings		33
Corporate debt securities
Fair value
Less than 12 months	$ 157,300	$ 171,154
12 months or longer	86,586	31,113
Total	243,886	202,267
Unrealized losses
Less than 12 months	6,866	1,585
12 months or longer	2,688	339
Total	$ 9,554	$ 1,924
No. of holdings | Holdings	635	331
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 777	$ 4,156
12 months or longer	1,618	7,064
Total	2,395	11,220
Unrealized losses
Less than 12 months	6	29
12 months or longer	26	95
Total	$ 32	$ 124
No. of holdings | Holdings	3	11
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 17,624	$ 10,836
12 months or longer	16,997	11,984
Total	34,621	22,820
Unrealized losses
Less than 12 months	175	85
12 months or longer	412	612
Total	$ 587	$ 697
No. of holdings | Holdings	30	19
Collateralized debt obligations
Fair value
Less than 12 months	$ 55,246	$ 21,598
12 months or longer	1,248	0
Total	56,494	21,598
Unrealized losses
Less than 12 months	826	41
12 months or longer	4	0
Total	$ 830	$ 41
No. of holdings | Holdings	39	12
Other debt securities
Fair value
Less than 12 months	$ 8,213	$ 1,499
12 months or longer	0	0
Total	8,213	1,499
Unrealized losses
Less than 12 months	11	0
12 months or longer	0	0
Total	$ 11	$ 0
No. of holdings | Holdings	7	1
Nonredeemable preferred stock | Financial Services Sector
Fair value
Less than 12 months		$ 9,644
12 months or longer		0
Total		9,644
Unrealized losses
Less than 12 months		75
12 months or longer		0
Total		$ 75
No. of holdings | Holdings		5
Nonredeemable preferred stock | Utilities Sector
Fair value
Less than 12 months		$ 1,093
12 months or longer		0
Total		1,093
Unrealized losses
Less than 12 months		25
12 months or longer		0
Total		$ 25
No. of holdings | Holdings		1