Investments (Unrealized Losses) (Details)
|
Dec. 31, 2016
security
|
| Debt securities: |
|
| Available-for-Sale Securities and Held-to-Maturity Securities [Line Items] |
|
| Gross unrealized losses, number of issues |
318
|
| Gross unrealized losses, percentage of total debt (in percentage) |
23.80%
|
| Gross unrealized losses, market value as a percent of amortized cost (in percentage) |
97.50%
|
| Gross unrealized losses, number of securities with maturities of 12 months or greater |
40
|
| Gross unrealized losses, number of securities with maturities of 12 months or greater (in percentage) |
12.60%
|
| Equity securities |
|
| Available-for-Sale Securities and Held-to-Maturity Securities [Line Items] |
|
| Gross unrealized losses, number of securities rated investment grade |
29
|
| Other-than-temporarily impaired securities, number of securities rated investment grade |
0
|
| External Credit Rating, Investment Grade | Debt securities: |
|
| Available-for-Sale Securities and Held-to-Maturity Securities [Line Items] |
|
| Gross unrealized losses, number of securities rated investment grade |
311
|