Just some quick numbers for 2015 for those that are interested. These numbers are based on my notes so nothing official from the company.

Jan 1st 2015 - Pilots on seniority list (3,411) Dec 31st 2015 (3850) Net gain of 439 pilots.

Attrition for 2015 - 219 CA and 295 FO total of 514 pilots. Average per month - 43 (18 CA, 25 FO)
I only had 6 months of data for 2014 but from July to Dec we had 106 CA and 156 FO for a total of 262. Average per month of 44 overall (18 CA and 26 FO)

I found it interesting that the numbers were pretty similar between the last part of 2014 and 2015. Not sure where the company is getting 20-25 but maybe they are only counting FO's? It would be interesting to run the numbers to see what bases are losing the most (MSP and ORD seem to be the most with a quick glance) and what airframe has the most attrition. Anyways, if any of you find it interesting like I do, I thought I would pass along the info.