What’s the best way to express discounts on an order summary?

Is there a standardized way to express discounts or credits in an order summary?

Example...

Sub total: $20
Discount:  -$10
Total:     $10

I've seen discounts expressed as -$10, $-10, ($10), and +$10.

Are any of these expressions more "correct" than the other? Is there a common or standardized way to express discounts? Where would these standards be documented?