Investments - Aging of Unrealized Losses (Details)
$ in Thousands |
Jun. 30, 2018
USD ($)
security
|
Dec. 31, 2017
USD ($)
security
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Number of securities, less than 12 months | security |
292
|
153
|
| Fair value, less than 12 months |
$ 524,737
|
$ 224,694
|
| Unrealized losses, less than 12 months |
$ (10,199)
|
$ (1,550)
|
| Number of securities,12 months or greater | security |
41
|
64
|
| Fair value, 12 months or greater |
$ 83,597
|
$ 114,243
|
| Unrealized losses, 12 months or greater |
$ (4,707)
|
$ (2,173)
|
| Number of securities, total | security |
333
|
217
|
| Fair value |
$ 608,334
|
$ 338,937
|
| Unrealized Losses |
$ (14,906)
|
$ (3,723)
|
| U.S. Treasury securities and obligations of U.S. government agencies |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Number of securities, less than 12 months | security |
14
|
16
|
| Fair value, less than 12 months |
$ 29,453
|
$ 29,806
|
| Unrealized losses, less than 12 months |
$ (1,187)
|
$ (394)
|
| Number of securities,12 months or greater | security |
8
|
26
|
| Fair value, 12 months or greater |
$ 14,657
|
$ 34,882
|
| Unrealized losses, 12 months or greater |
$ (873)
|
$ (587)
|
| Number of securities, total | security |
22
|
42
|
| Fair value |
$ 44,110
|
$ 64,688
|
| Unrealized Losses |
$ (2,060)
|
$ (981)
|
| Municipal debt securities |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Number of securities, less than 12 months | security |
27
|
21
|
| Fair value, less than 12 months |
$ 52,276
|
$ 38,628
|
| Unrealized losses, less than 12 months |
$ (811)
|
$ (264)
|
| Number of securities,12 months or greater | security |
10
|
10
|
| Fair value, 12 months or greater |
$ 17,670
|
$ 17,945
|
| Unrealized losses, 12 months or greater |
$ (671)
|
$ (395)
|
| Number of securities, total | security |
37
|
31
|
| Fair value |
$ 69,946
|
$ 56,573
|
| Unrealized Losses |
$ (1,482)
|
$ (659)
|
| Corporate debt securities |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Number of securities, less than 12 months | security |
209
|
94
|
| Fair value, less than 12 months |
$ 363,399
|
$ 128,313
|
| Unrealized losses, less than 12 months |
$ (7,575)
|
$ (829)
|
| Number of securities,12 months or greater | security |
19
|
23
|
| Fair value, 12 months or greater |
$ 45,034
|
$ 48,978
|
| Unrealized losses, 12 months or greater |
$ (3,125)
|
$ (1,129)
|
| Number of securities, total | security |
228
|
117
|
| Fair value |
$ 408,433
|
$ 177,291
|
| Unrealized Losses |
$ (10,700)
|
$ (1,958)
|
| Asset-backed securities |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Number of securities, less than 12 months | security |
42
|
22
|
| Fair value, less than 12 months |
$ 79,609
|
$ 27,947
|
| Unrealized losses, less than 12 months |
$ (626)
|
$ (63)
|
| Number of securities,12 months or greater | security |
4
|
5
|
| Fair value, 12 months or greater |
$ 6,236
|
$ 12,438
|
| Unrealized losses, 12 months or greater |
$ (38)
|
$ (62)
|
| Number of securities, total | security |
46
|
27
|
| Fair value |
$ 85,845
|
$ 40,385
|
| Unrealized Losses |
$ (664)
|
$ (125)
|
| Short-term investments |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Number of securities, less than 12 months | security |
0
|
|
| Fair value, less than 12 months |
$ 0
|
|
| Unrealized losses, less than 12 months |
$ 0
|
|
| Number of securities,12 months or greater | security |
0
|
|
| Fair value, 12 months or greater |
$ 0
|
|
| Unrealized losses, 12 months or greater |
$ 0
|
|
| Number of securities, total | security |
0
|
|
| Fair value |
$ 0
|
|
| Unrealized Losses |
$ 0
|
|