Methylates (mono and asymmetric dimethylation) the guanidino nitrogens of arginyl residues in some proteins. Essential for regulating flowering time. L-arginyl-[protein] + 2 S-adenosyl-L-methionine = 2 H(+) + N(omega),N(omega)-dimethyl-L-arginyl-[protein] + 2 S-adenosyl-L-homocysteine Ring-like homodimer. May be due to a competing acceptor splice site. Belongs to the class I-like SAM-binding methyltransferase superfamily. Protein arginine N-methyltransferase family.