Investments - Fair Values and Gross Unrealized Gains and Losses (Details) - USD ($)
$ in Thousands |
Dec. 31, 2019 |
Dec. 31, 2018 |
| Debt Securities, Available-for-sale [Line Items] |
|
|
| Total investments |
$ 1,113,779
|
$ 924,987
|
| Gross unrealized gains |
28,579
|
1,987
|
| Gross unrealized losses |
(1,418)
|
(15,484)
|
| Fair value of securities |
1,140,940
|
911,490
|
| U.S. Treasury securities and obligations of U.S. government agencies |
|
|
| Debt Securities, Available-for-sale [Line Items] |
|
|
| Total investments |
48,203
|
48,171
|
| Gross unrealized gains |
784
|
35
|
| Gross unrealized losses |
(58)
|
(1,376)
|
| Fair value of securities |
48,929
|
46,830
|
| Municipal debt securities |
|
|
| Debt Securities, Available-for-sale [Line Items] |
|
|
| Total investments |
189,530
|
92,014
|
| Gross unrealized gains |
1,721
|
206
|
| Gross unrealized losses |
(1,035)
|
(963)
|
| Fair value of securities |
190,216
|
91,257
|
| Corporate debt securities |
|
|
| Debt Securities, Available-for-sale [Line Items] |
|
|
| Total investments |
661,719
|
554,079
|
| Gross unrealized gains |
23,373
|
847
|
| Gross unrealized losses |
(211)
|
(11,688)
|
| Fair value of securities |
684,881
|
543,238
|
| Asset-backed securities |
|
|
| Debt Securities, Available-for-sale [Line Items] |
|
|
| Total investments |
170,153
|
171,990
|
| Gross unrealized gains |
2,603
|
792
|
| Gross unrealized losses |
(114)
|
(1,457)
|
| Fair value of securities |
172,642
|
171,325
|
| Total bonds |
|
|
| Debt Securities, Available-for-sale [Line Items] |
|
|
| Total investments |
1,069,605
|
866,254
|
| Gross unrealized gains |
28,481
|
1,880
|
| Gross unrealized losses |
(1,418)
|
(15,484)
|
| Fair value of securities |
1,096,668
|
852,650
|
| Short-term investments |
|
|
| Debt Securities, Available-for-sale [Line Items] |
|
|
| Total investments |
44,174
|
58,733
|
| Gross unrealized gains |
98
|
107
|
| Gross unrealized losses |
0
|
0
|
| Fair value of securities |
$ 44,272
|
$ 58,840
|