By default, IntelliJ IDEA shows Quick Documentation in a popup; to view it in the tool window, click in the popup and choose Open in Documentation Tool Window or press Ctrl+Q twice .
Switch to the previous or go to the next documentation page (for example, after clicking hyperlinks).
On macOS, you can also use the three-finger right-to-left and left-to-right swipe gestures.
F4
Navigate to the source code of the symbol whose documentation is currently open.
Open as PopupCtrl+Q: show code documentation in a popup instead of the tool window.
Auto-update from Source: when this option is enabled, the content of the tool window changes as you move the mouse pointer in the editor.
If the Show on Mouse Move option is disabled, place the caret at the symbol and press Ctrl+Q to see its documentation in the tool window.
Adjust Font Size: display a slider for changing the font size.
View Mode: by default, the tool window is attached to the edge of the main window and is always visible. You can change the view mode, for example, to make it visible only when active or to detach it from the tool window bar.
Move to: select where to attach the tool window.
Resize: adjust the size of the tool window.
Remove from Sidebar: close the tool window. To reopen it, select View | Tool Windows | Documentation from the main menu.
in the popup and choose Open in Documentation Tool Window or press Ctrl+Q twice . 
