As you do not indicate the power level, your detector can be either a simple diode with a 50-Ohm matching resistor at input, and a 100 kOhm load resistor at output. Such detector would need -10 to 0 dBm to operate well.
If you need to detect power levels from < -40 dBm, there are many "logarithmic" detector ICs by Analog Devices (AD83xx series), Maxim, Linear Devices, and Hittite.
I used some of them, AD83xx series often works from almost DC to >1 GHz. Check the specifications and request samples.
Concerning the differential signals, the AD83xx series has differential inputs; otherwise use a resistor voltage divider to get a single-ended input signal.