Commercial banks and thrifts continued their years-long retreat from the business of servicing home mortgages for other investors, and some observers are questioning whether nonbanks are strong enough to pick up the slack. As of the end of 2015, banks and thrifts serviced $4.054 trillion of single-family mortgages for other investors, according to a new Inside Mortgage Trends analysis of call-report data. That was down $84.3 billion, or 2.04 percent ... [Includes one data chart]