Investment Securities and Fair Value Measurements - Investment Securities (Details) - USD ($)
$ in Thousands |
3 Months Ended |
12 Months Ended |
Mar. 31, 2018 |
Dec. 31, 2017 |
| Available-for-sale Securities, Amortized Cost Basis [Abstract] |
|
|
| Total available-for-sale investment securities |
$ 59,618
|
$ 52,410
|
| Gross Unrealized Gains |
1
|
1
|
| Gross Unrealized Losses |
(358)
|
(210)
|
| Estimated Fair Value |
$ 59,261
|
$ 52,201
|
| Maximum contractual maturity period |
36 months
|
36 months
|
| Amortized Cost |
|
|
| Due in 1 year or less |
$ 37,410
|
$ 29,850
|
| Due after 1 year through 3 years |
22,208
|
22,560
|
| Total available-for-sale investment securities |
59,618
|
52,410
|
| Estimated Fair Value |
|
|
| Due in 1 year or less |
37,290
|
29,800
|
| Due after 1 year through 3 years |
21,971
|
22,401
|
| Total available-for-sale investment securities |
59,261
|
52,201
|
| Corporate bonds |
|
|
| Available-for-sale Securities, Amortized Cost Basis [Abstract] |
|
|
| Total available-for-sale investment securities |
37,976
|
38,383
|
| Gross Unrealized Gains |
0
|
0
|
| Gross Unrealized Losses |
(299)
|
(166)
|
| Estimated Fair Value |
37,677
|
38,217
|
| Amortized Cost |
|
|
| Total available-for-sale investment securities |
37,976
|
38,383
|
| Estimated Fair Value |
|
|
| Total available-for-sale investment securities |
37,677
|
38,217
|
| Agency securities |
|
|
| Available-for-sale Securities, Amortized Cost Basis [Abstract] |
|
|
| Total available-for-sale investment securities |
11,034
|
11,045
|
| Gross Unrealized Gains |
1
|
0
|
| Gross Unrealized Losses |
(52)
|
(42)
|
| Estimated Fair Value |
10,983
|
11,003
|
| Amortized Cost |
|
|
| Total available-for-sale investment securities |
11,034
|
11,045
|
| Estimated Fair Value |
|
|
| Total available-for-sale investment securities |
10,983
|
11,003
|
| Certificates of deposit |
|
|
| Available-for-sale Securities, Amortized Cost Basis [Abstract] |
|
|
| Total available-for-sale investment securities |
1,986
|
2,982
|
| Gross Unrealized Gains |
0
|
1
|
| Gross Unrealized Losses |
(2)
|
(2)
|
| Estimated Fair Value |
1,984
|
2,981
|
| Amortized Cost |
|
|
| Total available-for-sale investment securities |
1,986
|
2,982
|
| Estimated Fair Value |
|
|
| Total available-for-sale investment securities |
1,984
|
$ 2,981
|
| Treasury securities |
|
|
| Available-for-sale Securities, Amortized Cost Basis [Abstract] |
|
|
| Total available-for-sale investment securities |
8,622
|
|
| Gross Unrealized Gains |
0
|
|
| Gross Unrealized Losses |
(5)
|
|
| Estimated Fair Value |
8,617
|
|
| Amortized Cost |
|
|
| Total available-for-sale investment securities |
8,622
|
|
| Estimated Fair Value |
|
|
| Total available-for-sale investment securities |
$ 8,617
|
|