Investments - Fair Values and Gross Unrealized Gains and Losses on Investments (Details) - USD ($)
$ in Thousands |
Sep. 30, 2016 |
Dec. 31, 2015 |
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
$ 628,209
|
$ 564,319
|
| Gross Unrealized Gains |
13,713
|
560
|
| Gross Unrealized (Losses) |
(350)
|
(5,644)
|
| Fair Value |
641,572
|
559,235
|
| U.S. Treasury securities and obligations of U.S. government agencies |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
57,338
|
84,968
|
| Gross Unrealized Gains |
552
|
4
|
| Gross Unrealized (Losses) |
(13)
|
(490)
|
| Fair Value |
57,877
|
84,482
|
| Municipal debt securities |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
40,057
|
20,209
|
| Gross Unrealized Gains |
919
|
44
|
| Gross Unrealized (Losses) |
(43)
|
(174)
|
| Fair Value |
40,933
|
20,079
|
| Corporate debt securities |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
336,113
|
337,273
|
| Gross Unrealized Gains |
10,443
|
431
|
| Gross Unrealized (Losses) |
(227)
|
(4,377)
|
| Fair Value |
346,329
|
333,327
|
| Asset-backed securities |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
120,429
|
101,320
|
| Gross Unrealized Gains |
1,706
|
76
|
| Gross Unrealized (Losses) |
(66)
|
(603)
|
| Fair Value |
122,069
|
100,793
|
| Bonds |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
553,937
|
543,770
|
| Gross Unrealized Gains |
13,620
|
555
|
| Gross Unrealized (Losses) |
(349)
|
(5,644)
|
| Fair Value |
567,208
|
538,681
|
| Short-term investments |
|
|
| Schedule of Available-for-sale Securities [Line Items] |
|
|
| Amortized Cost |
74,272
|
20,549
|
| Gross Unrealized Gains |
93
|
5
|
| Gross Unrealized (Losses) |
(1)
|
0
|
| Fair Value |
$ 74,364
|
$ 20,554
|