It’s all in the documentation – and that’s taking the time, thus delaying a deal to unite San Francisco’s Orrick Herrington and Dewey Ballantine, which is anticipated to be completed in the New Year.

A deal to unite San Francisco’s Orrick Herrington & Sutcliffe LLP with Dewey Ballantine LLP is taking longer than expected.

Leaders of both law firms anticipated having partners vote on a merger this month. But preparing a partnership agreement has taken more time than expected, an Orrick spokesman said Tuesday. A vote will take place in mid-January at the earliest.

The deal is moving forward, but preparations to bring to the two large firms together can’t be completed before the end of the year, said Orrick spokesman Allan Whitescarver. The combined firm, to be called Dewey Orrick LLP, would have 1,500 lawyers in the United States and abroad and rank among the 15 largest U.S.-based law firms in terms of lawyers.

Both firms are finalizing the details of a partnership agreement, which spells out the responsibilities, benefits and organization of the merged firm. The firms also must finish work on other issues like real estate leases, tax implications of a merger and the impact of currency exchanges on revenue.

“There’s a lot of moving parts,” Whitescarver said.